There has been rampant speculation here that UA will never take delivery of the 350's.
Officially, UA still has the orders on the books, and as you mention it was converted to the 900 from the 1000. But I always thought the notable thing about changing the order was that UA increased the order from 35 to 45.
I kind of thought it would be 50/50 If they took delivery. But the new wrinkle/wild card is the Boeing max debacle going on right now (yes, I realize 737 narrowbodies are not 350's.... not remotely close). But I think Boeing has had a lackluster response and exposed some flaws that *could* be viewed as a precursor to the 777x project.
I see a couple of ways this could play out:
1. UA takes the 350s just so they have options if the 777x has issues.
2. Boeing recovers from the max issues and give UA another sweetheart deal because they need the sales.
3. Boeing scraps the 777x and builds something from the ground up.
4. UA sells its deliveries to another carrier or lessor.