it is not so easy if you want to replace the 3d models with asset bundles because there are scripts on them and you have to recreate all the monster manually by creating the monster then adding the script, nameing it correctly then put all in the correct bags. the bags are hidden.
if it were 3d models then you could simply exchange the obj files with a texteditor but with assetbudles it is not that easy.
a better way would probably be to change it in the json file but you have to see how is it done and what is the different between asset bundle and 3d model and even then i dont guarantee that it will work.
sorry it is not that easy. many things are involved ^^

I figured that much, I might try to just use .obj without animations if thats the case, I can use the asset bundles for the players (they work properly with the game setup and win/lose scenarios, since they just need the same name as the original ones), but for the monsters, static .objs can work