I took a longer look at Lahm's game in the first half of the Dinamo game - a great example of how 0-0's can be tactically so entertaining. Anyway, Lahm's role fascinated me to no end. A picture-heavy post is incoming, beware, as I feel the pictures illustrate my point really well.
I first noticed Rafinha's and Lahm's positioning during one attack where Lahm bombed forward through the middle with Rafinha staying behind to allow the defensive line to be manned by three players. This contrast between their positioning during build-up play is illustrated by the following picture.

Rafinha has brought the ball up, but remains central and passes it away at the slightest pressure, then retreats slowly backwards. All-in-all he got maybe 10 metres over the center line. Lahm, on the other hand, is drifting further up and more towards the center. We have seen this a lot this season, to be honest - Rafinha's role as the nominal leftback is to actually stay central and facilitate play for the other flank in a way that allows greater control over counterattacks.
Now, on to the next passage of play.

Bayern is building play and isn't wasting time. Lahm is charging towards the box. Were Kimmich Ronaldinho, Lahm might've been through on goal. Nevertheless...

Lahm's run has forced the Dinamo players further back and, more importantly, created space for Green to pick the ball up. Green is in a one-on-one situation right next to the corner of the box - a fit Robben could easily force this into a shooting opportunity or a deadly cross, but Green is still raw and (not incorrectly) opts for...

A run into the box and a pass to Lahm, who, not being an idiot, doesn't try to thread through a pass to Green who is shadowed by three players. Instead, he uses his low center of gravity to turn even further to the center. At this moment, there are 7 Dinamo players in the box and one covering Lahm - an overlapping run from deep from, say, Alaba or Vidal, would guarantee a powerful drive towards goal. Unfortunately, Alonso and Kimmich are unable to do this, so we're left with...

A high cross that Ribery brings back into build-up. Lahm, however, doesn't even think of going back to his nominal position of rightback. In fact, his movement in the next 10 seconds is vintage Francesco Totti, as he simply...

Drifts about in the middle waiting for an opportunity to either get the ball or create further havoc with his movement. I don't even know what to call this. An aggressive inverted fullback? A false wingback? I wish I had Müller to coin terms here...

To ensure that this role would not leave the team exposed, Green needs to track back consistently and effectively. Right here, you can see him tucked in, ready to deny Dinamo the chance to play the ball on the ground during the counterattack. Lahm, of course, stays central and closes off further passing avenues.

The pressing game works well. Lahm and Green shut off the possibility of switching flanks via the floor and Dinamo does not pick the aerial option, so as a result, Bayern quickly wins the ball back. Note Green's position compared to Lahm however - he is almost playing as a fullback! And it's a good thing that he does, because soon...

Dinamo threaten well. Lahm is in a position that Mesut Özil would occupy were he played out to the right, that is, quite centrally and ready to convert a loose ball or a quick pass into a counterattacking chance. Green, however, is flat in a back four.

He is even forced into a last-ditch tackle that stops Dinamo enough to force them into a tough shot from a nearly impossible angle. Lahm is next to Lewandowski, awaiting the recovery of the ball (which never comes). This is the most extreme situation I witnessed in the first half, but plays similar in principle happened almost every time Dinamo attacked down this flank.

Another example that isn't quite as easy to follow, not without a replay. Lahm has once again bombed into the box and has almost found himself offside. Kimmich however opts against a deep, in-swinging cross, and Lahm pulls back, taking a defender with him. This, however...

Leaves Green completely unmarked for another attempt at an in-swinging cross! Lahm's movement in the center occupies two defenders and allows two players nearly 40 metres from eachother to create a dangerous chance! This is Müller level off-the-ball movement, from a player that's, once again, nominally a fullback.

Just to drive the point about how often he ran into the box further home, here's him fighting for a cross with Dinamo's goalie Roberto.

Here Lahm is acting as chief attacking playmaker during a break. Unfortunately he doesn't have a lot of options to pick from, so the attacks slows down a bit. Not Lahm however, who...

Continues to charge forward into the center-forward position. Compare him and Ribery, and note this happened over five seconds. His drive to continue the tempo of the attack is commendable! But he is not a one-trick pony, so...

Realizing that his central position doesn't help the team with build-up around the box at the time, he drifts further outwards. Green and Lewandowski both take advantage of this to...

Move further towards the edge of the penalty area as Lahm, cleverly predicting his flank needed a man to stretch play, is found with a pass from Kimmich, I believe. Green continues to underlap Lahm, leaving him with...

Plenty of room to run through the middle. Dinamo, despite having seven players in the back, are unable to tackle him. Lahm plays a clever one-two with Rode and...

Having continued his run, finds himself, a mere three seconds later, unmarked in the box with the ball at his feet. Unfortunately he just barely could not find Lewandowski - had he, I'd be praising chance creation and a run that Messi would be envious of.
What is there to deduce from all this? Well, one thing is that the supposed "downturn" in Lahm's form is a fallacy. Philipp plays an entirely different role than before, but people expect to see more of the same. So do opposing players and coaches, by the way, but that line of thought is best reserved for when debunking the ridiculous "innovation for the sake of innovation" argument that is carelessly thrown around. He continued this role in the second half for periods, and found himself a metre away from Lewandowski in the box when the latter scored the opener, as well as took a shot himself. Müller's addition made his flank a lot more deadly than Green managed to, though. Lahm eventually drifted out wide to enable Müller to continue his flourishing partnership with Lewandowski, though.
Generally, the roaming fullback role is reserved for Alaba, with Lahm instead playing the safe, conservative role that Rafinha is in the pictures above. The fact that either fullback can switch on this clever system at will is a huge weapon in Guardiola's tactical arsenal.