What sort of changes and what tiered QFF member are you?
If it's major changes as you've alluded to, if you're Bronze and if there's still reward seats showing online that you wish your final itinerary to be, I'd suggest to cancel in full and then re-book online rather than attempting to change via the phone.
It's honestly really a lottery to change reward seats if you're anything less than Platinum as 80% of the time, consultants don't have the same visibility of reward seats that are showing online.