It appears that you can amend a GUFT booking until 24hrs before departure of both outbound AND inbound flight, and retain your upgraded cabin. The GUFT is lost if you cancel/amend inside the 24hr window. The amendment will, of course, be dependent upon the availability of I class if you have booked through an agent or U class if you booked directly with BA.

Getting this change done could prove to be a little complicated. Most travel agents are very wary of rules for GUFT bookings and I would expect many of them would refer to BA for assistance with rules. I had to do a bit of digging around myself as I was not sure of the answer. I was expecting standard MFU rules to apply to partflown bookings but I found an MS Excel file which clearly differentiates between MFU and GUFT bookings. You have unnecessary hassle convincing your agent or BA of this. Or maybe I have too little faith in my colleagues. So, for a booking which has a T class base fare, the £50 change fee should apply subject to availability of the upgrade,