Hey, that’s a pain. Unreal only lets you move assets forward, not backward, so you can’t drop a 4.12 uasset into a 4.8 project. If the pack came with raw FBX meshes and textures, grab those and import them into 4.8 yourself. If not, spin up a quick 4.12 project, export the arms as FBX, then pull them into your 4.8 game.
If you think you might upgrade the whole project anyway, make a full backup or branch first. Run the upgrade on the copy, test all your systems, fix any breakages, and only then switch your main build over. Good luck!