Migrating Files Due To Incompatibility Error

I wanted to use the Animation Starter Pack from the Marketplace when, to my surprise, it’s incompatible with the new 4.13. I instead looked to migrate the files from 4.12 to 4.13, but a load of errors popped up saying they had an incompatible file extension (.uasset). Is this correct, or have uassets been removed in this new update and replaced with a better (more efficient?) system?