The layout of the skin parts on the bmp is always going to be completely different on the new shape, try to use the old bmp on it and you'd have the upper left wing draped over the RH fuselage or whatever. So a new AH shape always means new skins, the only thing skinners can carry over being the research they did on the old ones.
Try to think of it as an opportunity Vraciu. The current P-51D skin slots are full and the B/C nearly so. If and when the P-51 series gets redone then once you have made a new template you (and other skinners) will have up to 32 skins slots available for each, plus maybe another 32 if HTC make an Allison Mustang variant. Also those subjects already skinned by players no longer in the game would be up for grabs. An AH3 P-51 shape will certainly be way more detailed, have all the new effects to work with, be in 2048 res and have no stretching issues etc.