Welcome to Flyertalk
When are you making the changes to your ticket? I've changed Miles for Upgrades (MFU in FT parlance) before, and had no issue with changing the dates. In the back of my mind I believe you have to do this more than 24 hours in advance of the first leg
and there has to be availability on the flight which you want to move to. No changes after departure tho....
I'm sure someone with much greater expertise will be along in a minute tho....