in my (single) experience, you lose the miles when changing an upgraded flight to another with availability if the miles were expiring but it is very possible that the experience differs on which agent you get or that the rules have changed.
Originally Posted by
father_ted
An interesting question is what happens if the original booked class is not available... say you booked a Q ticket with a $200 change fee and then upgraded with miles to D. Can you pay $200 and change to a flight where Q is closed, but D is available?
Again, in my few experiences, you need fare availability in the original booking class. To make a change, you need to cancel the original u/g, change and re-issue the ticket and pay any change fees etc, and then re u/g with miles.
Another random anecdote:
friend was flying IKA-DXB in Y. I changed his booking to J with miles as a surprise. Miles dedicated. He then went and changed his ticket with his TA for IKA-DXB without any idea the the ticket was in J [and apparently the TA never mentioned anything]. The upgrade stayed for the new flight IKA-DXB (re-issued ticket showed Business class) but miles were refunded. I don't know why/how this happened but maybe it tells us something about how the change happens (don't ask me what exactly it tells us haha)