A not so hypothetical situation:

A ticket needs to be changed to a different flight, as well as upgraded. Both flights have I inventory, but the original class of booking is not available on the flight to be changed to.

In this scenario, I'd imagine it makes sense to first upgrade the ticket, and then change flights. Would this work however? Or would they still charge the fare difference as upgrade by miles still limits you to fare rules of your original ticket? Anyone who has experience doing this?