I've hacked the odd square game in my time, including a bit of FFX, actually.
Reason dictates that it
can be easily done, with the help of a hacking device, such as ARMAX or codebreaker And if Square Did anything even remotely similar to KHII, you can use a number of ways to do things like this:
Most common ways are going to be a texture pointer swap, if there are pointers for Textures, and the much more easily done Model label swap. assuming the bones are the same for the variations of the characters, which it's probably safe to say they are.
I do believe though, that FFX models and animations are under the same File (.FFM, or .FFB or something) So If you can't swap textures or models directly, you can swap the (.FFx) files, but they'd have to have the same animations as the people you're trying to replace. and in actual fact, i remember I used jechts files in place of auron's, and well, lets not discuss that
anyway, in conclusion, it probably
is possible to make the aeons look like their dark counterparts.