Sorry to hijack the OP's post, but since my question is similar I didn't want to start a new one.
I have a (paid) economy ticket, purchased through Swiss. LAS-SFO on UA, SFO-ZRH-MAD on LX. I'm flying next week.
When I look at my booking on Swiss, I see this screen:

I know I'll have to check in online with UA, and I'll probably get some random assigned seats with LX - will I be able to change them online with Swiss after I do my UA check in? I have a 50 min layover in ZRH and while I know that's supposed to be enough time, I'd still like to be as close to the front of the plane as possible just in case we're delayed getting in. (And there's only one flight a day on LX to MAD!)
Thank you all!