There is a fundamental difference between AH1 and AH2 plane shapes when it comes to skinning them. AH1 plane art was originally created on 256 by 256 resolution tiles. Because this made detail very blurry HTC's artists overlayed small patches of higher detail where needed, i.e. for noseart. Also they duplicated areas wherever they could, for instance the left and right wings are sometimes the same part on the skin but mirrored. They also stretched large areas of the skin to save space, usually the top and bottom of the fuselage are stretched horizontally. When skinning these old shapes the tiles are now skinned in 1024 res tiles but the stretching, mirroring and patching issues still remain. There are also many bugs, distorted textures etc. that you don't get on AH2 shapes all of which makes them a pain to skin.
AH2 shapes are built on a single 1024 (or 2048) res tile and have few or none of the above issues. This means there are few schemes you can't do because of shape issues and the skins turns out looking a lot better. The later AH2 shapes are more detailed (higher poly count etc.) than the earlier ones but they are all way better in this regard than the AH1 ones. While there are shape errors on some 109s and Spits they are minor compared to the issues on the AH1 shapes, so I can't see those being redone until all the AH1 shapes have been brought up to scratch.
In general HTC seem to have upgraded the more popular MA rides to AH2 standards first, which makes sense as more players will benefit from the improvements more of the time. Sometimes they are redone to add a new variant (Ju-87G-2 etc.) and of the remaining AH1 shapes the Yak is the one that would offer the most choice of new variants.
