Called United...the agent I spoke to was not super clear in her explanation, but she basically said that anytime there's a schedule change with a partner airline, they will reissue the ticket (as you said, Lux Flyer). I know that a couple of the UA segments have changed a couple times without reissuing anything, but I guess I have never seen a partner schedule change before in any of my travels (at least, none that I can remember). Anyway, problem solved! Thanks!