cahgeing magic intro animation

The easiest and fastest way to make that change is probably just to use FF3usME. Just go to the battle editor tab and then the animations 1 tab. The white and black magic intros are in the drop down menu, you can just paste the bytes for the other intros into those spaces.

I'm not sure off hand where they are in the C banks if you wanted to do anything more in depth, I'd have to look into that when I have time. If you're trying to just change the grey magic one to make it unique from the white magic one that might take a bit more effort, and would definitely require locating that in a hex editor.