In most cases to use animations with a skeletal mesh in UE4 you use animation blueprints. When you change the skeletal mesh you’ll need to be sure that the animation blueprint also works with the new mesh. If the old one doesn’t work, you make another animation blueprint that works with it, and to set the new one you use:
“set anim instance class”
If you use basic animations - you use animation assets. They are good to use with assets that have a few animations and you are too lazy to make a proper animation BP for it (like a working fan). And in cases when you’ll want to change between animation blueprints and animation assets you use “Set animation mode”. (the character was a mimic and was pretending to be a fan with a single animation asset, and then transformed to a walking monster with animation blueprint)