For the Packers, the changes lie in personnel more than anything, because of the number of injuries the team has been forced to overcome. Those changes have had a significant impact on both sides of the ball.
Meanwhile, for the Bears, with the defense and special teams rather consistent and impactful all season, the adjustments have mostly been in their offensive approach as coordinator Mike Martz has adapted his scheme to what the Bears do best.
Here's a closer look at those key variations compared to Week 3:
Green Bay offense
On that Monday night at Soldier Field in late September, tight end Jermichael Finley caught nine passes for 115 yards, accounting for more than one-third of quarterback Aaron Rodgers' 316 yards on the evening. After the following week, Finley was on pace for a 1,200-yard campaign through one quarter of the season. But in Week 5, he was lost for the year with a knee injury and the Packers' passing attack needed to take on a different identity.
There was an adjustment period, but eventually Greg Jennings (72 receptions, 1,168 yards, 12 TDs) emerged as the go-to guy with a plethora of complementary pieces around him. Jennings, Donald Driver, James Jones and Jordy Nelson all have at least 40 receptions, the first time four Packers wide receivers have reached that mark in the same season. Add in a fifth 40-reception target in running back Brandon Jackson and the passing game has five players, regardless of position, with 40 catches for the first time in 30 years.
"That's why we're a team, it's a collective effort," Jennings said. "Obviously it doesn't take one guy to get it done, it takes a group effort, and that's what we pride ourselves in doing is everyone stepping up and making plays when their number is called."
Meanwhile, the Packers offensive line has probably been more stable than in recent years, but the one major change was rookie Bryan Bulaga taking over at right tackle for Mark Tauscher, who suffered a season-ending shoulder injury in Week 4.
In the first meeting with the Bears, the Packers and Tauscher admittedly had a tough time with Chicago defensive end Julius Peppers. He was a big reason running backs Jackson and John Kuhn combined for just 43 yards on 13 carries (3.3 avg.), and he drew a holding call on Tauscher that nullified a touchdown pass to Finley.
Now it will be up to Bulaga to handle him, most certainly with some help from the tight ends and/or backs. For the season, Peppers has eight sacks, two interceptions among nine passes defensed, and three forced fumbles. He's a nine-year veteran with the most complete game in Chicago's front four -- against the pass, he has the power to bull rush and the speed to get around the edge; against the run, he has the strength to hold the point of attack and the quickness to get penetration and be disruptive.
Coming off a week in which he faced Giants' double-digit sacker Justin Tuck, Bulaga has another order just as tall on his hands.
"He's got it all and he'll do it all," Bulaga said. "You can't overset or overdo one thing because he'll do the other. You have to use all of your tools and just play your game."
Green Bay defense
In some ways it seems as though the entire defense has been revamped due to injuries over the last three months, but perhaps the key to making so many changes work has been the solid play in the middle. The Packers have gone from employing Nick Barnett and Brandon Chillar for most of the snaps at inside linebacker to A.J. Hawk and Desmond Bishop with no noticeable drop-off.
"I've really got a lot of confidence in both A.J. and 'Bish'," defensive coordinator Dom Capers said. "Both those guys since I've been here, they've been through the ups and downs, play time (against) this team (or) that team. I think they really complement each other. I think A.J. has done an outstanding job out front of the huddle, making the calls and checks. I think we've seen Desmond's playmaking ability, that he brings some explosiveness. And I think they've been working very well together.
"I don't think there's many teams in the league that probably could go through what we've gone through at that inside linebacker and still get the kind of play that you're getting out of A.J. and Desmond."
That pair ranks 1-2 on the team in tackles, with Hawk adding three interceptions and Bishop three sacks.
The best illustration of how different Green Bay's defense will look against Chicago this time is to recap a number of significant plays from the first meeting. Safety Derrick Martin had an interception, linebacker Frank Zombo and defensive end Cullen Jenkins each had a sack, Barnett had an interception nullified by a penalty on Zombo, Chillar was called for pass interference on Chicago's final drive (yet tight end Greg Olsen still made a leaping 21-yard catch), and safety Morgan Burnett was flagged for pass interference later on that final drive to set up the Bears' game-winning field goal.
None of the six defensive players mentioned there will be on the field on Sunday for Green Bay. Zombo (knee) and Jenkins (calf) are both out with injuries, while the other four are on injured reserve. But through waiver-wire pickups, free-agent signings and continued growth of several young players, the Packers still rank second in the league in scoring defense and third in interceptions.
"We've played through a lot of different people playing a lot of different positions," Capers said. "I'm proud of the way our guys have responded, because there's never been an excuse when we've had an injury. The next week we've had a different guy line up in there, and everybody on the team expects them to go in and do their job. They know there's a sense of accountability on this defense.
"It's a great example that if you get a group of guys that are unselfish, and they understand their job and there's a sense of accountability of them taking care of their business, that there's nothing really that you can't accomplish. I think we've got that. I like the attitude, the approach, the unselfishness, and the guys understand that if everybody does their job, everybody's going to get a chance to make plays."
Analysts wondered from the beginning whether the union of Martz and his wide-open offense would work with quarterback Jay Cutler and the current Bears. The early results weren't the most promising, with Cutler suffering through a nine-sack game (which kept him out a week with a concussion) and a four-interception outing as the Bears struggled to a 4-3 record at their bye week.
But since then, Chicago is 7-1, and unlike some of their past playoff teams, the Bears aren't simply asking their offense not to screw up what the defense and special teams accomplish. This offense is producing, and doing so impressively, as Martz has found better run-pass balance.
"I think he's going through a normal process that happens to coordinators when they go into a new place," Head Coach Mike McCarthy said. "Sometimes it doesn't work out the way you want it during the beginning, but they've continued to improve on offense throughout the season and they're playing their best football right now."
The Bears made some adjustments up front, shifting right tackle Chris Williams to left guard, flipping Roberto Garza from left guard to right guard, and inserting rookie J'Marcus Webb at right tackle. They've also committed more to running the ball with Matt Forte, who has topped 90 yards on the ground in four of the past six games, compared to once through the first nine games of the year.
According to the Chicago Tribune, over the last eight weeks the Bears have run the ball 238 times and passed it 231, making them the only team in the league to run more than pass over that span. That's a 51-49 run-pass ratio.
Contrast that with Cutler's first six games (he missed Week 5 with that concussion), when the Bears ran the ball 114 times and passed it 196, a 37-63 ratio. Cutler had seven touchdown passes against seven interceptions over that time and has posted 16 TDs with just seven interceptions since.
"I think they know what they want to do now," Bishop said. "I think they know how they want to attack, and they stick to what they do and they're getting really efficient at it. I think early on it was here and there, doing different personnel (groups), but I think Week 16, they are who they are."
And that's an offense taking advantage of everything the defense and special teams do for it, particularly in the area of field position.
"All you have to do is look at the game last week against the Jets," Capers said. "They're down by seven at halftime, and they go out and within nine plays they've scored three touchdowns. They took the ball over on the 32, the 40 and the 49. They go 32 (yards), one play, touchdown; 40, three plays, touchdown; 49, five plays, touchdown. There you go, it's a whole different game."
Sunday's late-afternoon rematch will clearly be a whole different game, too, whether or not the playoff stakes for Chicago change as a result of other NFC scores earlier in the day.
The Packers have endured and survived to give themselves a chance, while the Bears are simply waiting to see exactly how their postseason will begin.
"We've had an up-and-down season," Rodgers said. "We've overcome a lot of challenges, a very resilient group of guys in this locker room. I'm very proud of the number of men who have stepped up and played a big role for us. Some men who we didn't really count on at the beginning of the season, we didn't think they were going to play as a big a role as they have. But I'm very proud of those men and the obstacles we've had to overcome.
"Them? They're kind of on a roll. They've been playing pretty consistent most of the season. Had a couple hiccups, but they beat a good Jets team and beat the Eagles as well. They're playing very well, wrapped up the division, and we'll see what happens Sunday."
Additional coverage - Dec. 31