I started programming with UE4 about a year ago and bought (and downloaded for free) quit some stuff now but I descided to change to Unity because my Friends know that one better. I just wanted to know if there is any way to use all the Assets i bought in Unity too, since if i open the Assets in the Explorer it only shows them as .uasset.
Without exported and re-importing, no.
Additionally, unless specifically state, you should contact the author of assets you wish to transfer to make sure that you are covered.
At least I would, just to cover my butt 
If its from the marketplace, you are not allowed to use Epic Games content in Unity. Non-Epic is fine to convert. Just right click the asset in the content browser and choose asset actions->export.
Tell your friends to move to Unreal… it is the only right thing to do.
Agree!
Thanks to Unity team for showing the way a game engine shouldn’t be 
If the game is simple and perhaps on mobile then Unity would suffice but if it is a multiplayer 3D game then they seriously should consider Unreal Engine instead. They don’t know what they are missing out on.
Or maybe try my converter https://gumroad.com/l/unrealtounityIt exports in either OBJ or FBX and can export static meshes, lights, LODs, collision meshes, materials with shaders.