Upgrading One Way

Made a call to upgrade my economy flight back from Orlando to premium yesterday and the booking agent said I would have to upgrade both ways due to the fare category. It's never been an issue before but is this correct? It's a full cash fare not using any miles I have currently economy both ways.

never heard that one before, when i've upgraded at check-in i've often wanted both ways but usually only get the leg i'm on at the time, tbh if offered both ways i'd take it!

which fare code?

toolbox_fare_codes.php

Just a good deal £300 for 3 of us return leg but £1000 both ways still good was working out £2380 return for 3 pax. Very tempted.