I doubt EDI will see any UA 787s, I heard they will be used primarily on their trans Pacific routes, however it's still a while yet until they will actually be flying with UA.
I know economy is more often verging on, if not totally full, but is J class a consistent earner for them from EDI? UA won't be willing to send over a 777 with 50 odd seats up front if a 757 with 16 isn't being filled up on a regular basis.
The 767, perhaps would be a good aircraft during the summer if they do not bring back the double daily. It has 25 BF seats, so a good compromise, as well as the additional Y class seats. Delta managed a 767 on a daily basis, so the terminal can cope. One thing though, I beleive the 767 cabins are not as modern as the others, especially in BF with no lie flat seats, which is pretty poor by today's standards.