Changing model fbx with existing animation applied for it

I have animated character lets say mario and i want to change the model but the animation still applies for the new model for example if i have the run animation it should work after i changed the model of mario to wear shorts
how can i do that
if i can change the fbx file does the animation be worthless?

These are animations that you have made within Unity, correct?

In that case, they reference the child transforms of the model root by name. Which means that as long as the new model has the same bones with the same internal relations as the old one had, you should be good.