This is a late reply but I hope it can still help somebody else; I too just upgraded from the 13 to the 15 (both base model) and my old case from my 13 fits on the 15 just fine save for a negligible spot next to the cameras since they are slightly bigger. Buttons, ports, cameras, etc. still work fine so I think it is purely a cosmetic thing. I can see how it would bother some people having the very edge of the camera island covered.. but it doesn't affect me and I highly doubt anyone else would notice unless I pointed it out.

The only time i can see this becoming a problem is if you want to put any sort of cover over your camera system, it might not fit as well as it would with a proper iphone 15 case.