My guess is if the MMA aircraft is what UA wants they can keep the 350 order or replace narrowbodies with 320/321. But, if the MMA doesn't look like it would work well for UA, then they can convert the 350 orders to the 321LR, if it is proved to work well as a 757 replacement.
Who knows what the MMA will be. Narrowbody to replace 757, a 767 looking aircraft that has good cargo capability, something completely different.
I can't think of why an airline would want something like the 762, stubby twin aisle with maybe 7 or 8 across. Efficiency is with 6 across single aisle or at least 8 across twin aisle with standard LD3 containers.