If they changed the skeleton, I think the only solution is to manually retarget the old skeleton to the new one. If they didn’t changed the bones hierarchy it should be a simple task (the linked documentation has several how-tos).
Well… with our plugin!
If you want to make your new Mixamo character compatible with the standard UE4 skeleton, you can use our plugin. It’s just a click on the Mixamo skeleton, then you’ll be able to retarget their animations. Check the video at the beginning of this thread to better understand what the plugin does: v=OSaYNLdrYck .