How do I change the models of characters that already exist in the game?

Which specific files are you talking about? I knew everything you mentioned, and I opened both xdb files and data files; I even managed to create retextures (and don't think I'm not grateful that you're explaining this to people who don't know), but I couldn't figure out how to change the models themselves. I thought I needed to change the paths in gameMechanics-creaters-haven-angels, but it didn't work.

You need to find the file GameMechanics\Creature\Creatures\Haven\Angel.xdb

in it, there is a line/tag Visual

{Visual href="/GameMechanics/CreatureVisual/Creatures/Haven/Archer.xdb#xpointer(/CreatureVisual)"/}

replace it with

{Visual href="/GameMechanics/CreatureVisual/Creatures/Neutrals/Death_Knight.xdb#xpointer(/CreatureVisual)"/}

the files should contain Less than < and Greater than > signs, i.e., not curly braces { }, the forum just processes them according to web design rules...

as a result, the angel will be displayed with the Death Knight model...

but the question is, why would you need to do this?! What is the final goal of such a replacement?!