It used to be like that in the NAEC for upgrades (this was back when the NAEC was the only place you could actually get upgrades on BA). Was pretty neat as you'd get the points/miles for J/F (fare was booked in D/A), rather than the booked class. Plus of course back then it was a straight Y-J upgrade. I don't remember now how many miles it cost but IIRC by the time you'd flown, the 'net' cost in miles was pretty low. Cash cost was good too as that was pre-WT+, so the u/g was Y direct to J.

This would be a useful perk for Golds if they restored it. I can see how it would be chaotic/lossmaking if they generalised it.