A belated thanks to all for the help. In the event I was able to change with no fee and no fare change.

I did want to share an experience on the quoted suggestion, however. I tried to do this, but the website gave me an error message and told me to call in when I clicked the "change booking" link on the itinerary in MMB. I suspect the problem was that I had waitlisted for an upgrade award on the flight I was changing.

Anyway, after less than five minutes on hold with MPO (during which I had to listen to a couple recorded messages about how doing things online can save you time on the phone -- ha!) I was able to change the flight and waitlist for the ug on the new flight.