What is 10 hours in the context of a once in a lifetime trip? A heartbeat, an eye-blink?

If you can change your return before you begin travel then you'll be good to go. If not then just enjoy the new LH First Terminal, and maybe ask LH in FRA if you can go out on the earlier LH flight. I doubt UA will entertain your request in FRA. If you have checked bags then you'll have to deal with that, claim, re-check, etc.