Not sure if this is similar but I tried to do something similar recently. I wanted to change the date of my inbound flight and change from wt to wtp.

The agent after speaking to her supervisor couldn't do this as one transaction. She would have to change the date of the inbound first, then price up the upgrade. Each would attaact a separate £150 change fee.
If there had been no availability on the first step to price up the date change, would she then have not been able to do part 2.

I wonder what would happen if they try to do it as upgrading to F on your current date first and then change the dates.