I'm confused. I think you're saying a Dewalt 12 Ah PTB is 3 x 72 W-Hr individual packs inside.
If so, then going from a 12 Ah to a 15 Ah would make them 3 x 90 W-Hr packs, wouldn't it?
Regardless, I understand that having PTBs marked as TSA compliant at their original capacity, but flying with them knowing they are a higher capacity than the case is marked could still be a problem. That said, if they're still under 100 each, and made with the original shells, well, would anyone ever be able to tell that they are not the original capacity?