What you could do is have the early war IS-2 the buildable one, and a late war IS-2 as a replacement for the IS-3 And you could give the IS-2 ace those white stripes around the turret, so you can see the difference between em I think that would fit pretty good.
Basically the IS2 ace will be the exact same as the current IS3, just using a different model though
I can't wait till they add british to CoH:O; the extreme forces leveled by the new imbalances will create a black hole around the servers that destroys half of Canada.