Trust me I had a hard time even thinking of doing this, but since the steelbooks have no markings (that I can see) on whether it's BD or UHD, I just did it. Also, in my neck of the woods, the scalpers buy out all the steels at Best Buy if you're not quick enough, so you can't get your hands on them. I already had steels for Captain America 2 and 3 but not the first one, so I used those. Wanted the UHD steel of the first but missed out completely because they sold out too fast. Now I'm left with an unmatched set and I'm not paying scalper prices for the first steel. Same happened for Avengers. Already had the steels for 2 on BD which are embossed and really nice, so I just swapped the discs and added what I wanted in them (3D versions with UHD).