It's possible but not really easy. I know that you can change Mario's textures but changing the whole model is a lot harder. You can probably find it somewhere in the files maybe try to decompile the file.
If you're gonna decompile it then do it with a copy of your rom it would be a shame if you lost the whole rom
maybe you canfind more information online because I have seen a lot of character replacing so there is probably someone who made a tutorial on it
this is what I found:
and maybe the rom manager or model manager has this feature. If you don't have these programs then I could link you to them if you want.
I hope this helps you a little bit