Originally Posted by Peter Snijder

I’m in economy.
The changes (change of time and change of carrier) led to a new UA flight number, and I believe there was then a revalidation since the ticket number hasn’t changed on the UA website. So I believe it’d be another revalidation (not reissue) if I change to the earlier flight.

Edit: I called UA, and not only did I get an agent immediately (), but she was able to make the change without any issues. The UA website showed XN class availability, so I knew there was a good chance, but you never know…
She booked that flight in HN class, not sure if that matters.
The remaining flights are left unchanged. No new ticket number, so I assume that means revalidation, not reissue. It’s the same ticket number as before the involuntary rescheduling this week.
The OS website shows the updated itinerary and the correct ticket number.