Classic Metal Sonic over Modern Sonic? Sure, why not? Metal Sonic DOES "boost" instead of roll, so I guess it makes sense.
Anyway, I saw that model swapping had just been figured out by this awesome Russian modder, Niko Bellic:
http://www.youtube.com/user/MrHelias94
and I decided to give it a shot myself. I'm actually quite pleased at how it came out, although it still has quite a few bugs that need to be worked out.
Also, I apologize for the random freezes in the video, my computer has been acting up lately, so I'm probably going to have to figure out how to fix it at some point...
Enjoy!
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