United’s Star Alliance award rules have been revised slightly, in a positive way.
It used to be that awards involving United’s Star Alliance partners could not be changed at all, in any way (at least officially, though sometimes it could be done at the airport by other carriers).
Now United will allow you make changes to an award as though it were an all-United itinerary if the only remaining flights in the itinerary are United’s. (The rule actually appears to say any United segment could be changed even after travel commences, but I’ll want to see this work in practice before stating definitively that it’s accurate.)
For example, if you wanted to fly United from Chicago to London and back, but those flights weren’t available and you had to fly United to Frankfurt, connecting on Lufthansa to London. You’d have taken an all-United itinerary if you could (if coach, for economy-plus.. if in a premium cabin perhaps because of the greater flexibility in making changes). But those flights weren’t available. So you include a partner in the itinerary, and had to give up the ability to make changes. Now at a minimum you’d be able to change the last flight segment which is on United.