Yes it very probably would require some redesign. The vast majority of other laptops that get to 32GB of RAM use so-DIMMS and not soldering planar RAM chips to the logic board. So the 15" would have to adjust to that. That solution requires ripples through the case and to the motherboard. That would meet the criteria of major redesign.
If try to do it without so-DIMMs than it will means expanding the logic board significantly and shrinking the batteries. ( which I doubt would be a popular overall decision as life time would go significantly down). Trying to solder 32 GB of RAM onto a planar surface is waste of volume. There would be a penalty for that.
Where it would not be redesign is if it didn't exist yet.

There was no 2016 15" function key model. In that context, technically it isn't redesign.
What would not be surprising would be to see Apple kill off the 2015 models and to pair the "fcn key" 13" 2016 case design with a matching 15" 2017 design that was thicker, had function keys, and a USB 3 Type A port ( and maybe HDMI and/or MagSafe ). Also, maybe some somewhat minor tweaks to the 13" MBP fcn key model to get a Type-A port on the other side ( so had 3 + headphone jack). They would need to tweak the speaker assembly, but two ports is kind of a low amount for a MBP. Lower the prices of the function key models into the slots held by the 2015 stuff.
In short, fill out and update the "function key" options ( only 13" got updated in 2016), and keep the higher tier of touch bar. ( similar how had full regular screen line up just after had high tier Retina line up... before Retina went 'mainstream' in the MBP line up.) At least one, if not both, of the 2015 models will probably get tossed. ( the comatose MBP 13" 2012 stuck around for a long time (~4 years) , so maybe the 2015 13" will take its place in historical legacy offering. )
The 2016 13" function key model looks like a hijacked MBA 2016 project. Apple needed a 'safe' design to roll out along side something that might not high the deadline. There was no 'safe' 15" option. Therefore Apple held the 2015 models in the line up. Fix those two things properly and Apple can do with the normally do and "let go" of the older models.
Apple could just "speed bump" the touch bar models to gen 7b (Kaby lake) with no other tweaks ( except for minor corrections ).