I always find it strange when companies effectively stop you spending more money (in this case £39) by adding extortionate admin/change fees.

If you are changing nothing else, i.e not changing the flight time or air export legs, purely upgrading, they should waive the admin fee. THey are shooting themselves in the foot.

I've got a similar situation on a flight to the US. I'm tempted to upgrade from WT+ to Y, I dont' mind so much the c. £450 upgrade cost, but I refuse to pay the £150 change fee, so looks like I'll be sticking with WT+.