By replacing every single .dcc file (and maybe the .cof files, though I'm not sure) used by that character.Let's say you want to make the sorceress look like the necro (I know, you probably intend something more creative...this is just for an example).
You would extract the animations for the necro from the .mpq file. You would rename them to bear the names of the equivalent sorc animations. You would take those extracted and renamed files and insert them into the patch.
The sorc would now look like a necro.
This is a lot of work...as there are dozens upon dozens of animations for each character. It would also make your patch *HUGE* so if you want anyone else to play it...you might want to forget it. Few people want a mod that takes hours to download. 
More importantly, if you want entirely new animations for the character, you are in for hundreds of hours of work, probably. You'll need a 3D modelling program, lots of skills, and lots of patience.
And if you were hoping to change a character into a monster (thereby capitalizing on the existing monster animations), you will run into several problems, not the least of which is that the monster will not have animations to cover all of the armor, weapons and other animations used by characters.
You could use the CV5 program to convert Diablo 1 character animations for use as D2 character animations...but again, it's a lot of work and your patch would be huge.
So, it's possible...but the question is whether you still think it's worth the work. 
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- JEFF
"A hero is no braver than anyone else. He is only brave five minutes longer."
For my latest mod (for LoD/expansion), download at http://www.fileplanet.com/dl/dl.asp?/planetdiablo/phrozenkeep/mods/sic_beta.zip - Or for old stuff (for classic D2), visit my orignal mod site at www.planetdiablo.com/sanctuary
[This message has been edited by jbouley (edited 25 November 2001).]