A heat exchanger is not a heat exchanger. Each one needs to be sized and optimized for the application, together with the associated pumping and piping systems. Unless you've done the heat transfer calcs, then I'll assume that they need to revise/replace it. I think it's far more likely that they can use the one directly out of the 3, but I may be wrong on that. Further, the software will need to be revised, and perhaps some of the control electronics.
Sure, everything that you say is possible. They could have as many versions of the mounting structure, drive train, software and electronics as they cared to build, at whatever it cost. But you've not given me a single reason why they would. I think you'll agree that they right way to do this is to do it once - and not do part of of the job now and part later.
Revise the entire drive train in one shot, including the pack, motors, exchangers, etc... That gives you a clean cutoff where you don't have a bunch of half-upgraded cars with intermediate components and intermediate software
And further to the point, it's not like people have been clamoring for a new motor. I've never seen one post that is hounding Tesla for that. Yes you might get another 10-15 miles out of a slightly more efficient motor, and maybe it save a few dollars but that's not really enough impetus to create these half-upgraded cars. People have, however, been clamoring for a new and larger pack and for a track mode. I suspect the new drive train is intended to deal with both of these issues. And I suspect that there's more savings to be had in replacing the whole shot with a version of the 3 drive train.
