I've just had to pay a £25 per pax change fee upgrading an Avios booking from Club to First, in addition to the increase in taxes. It was a last moment add-on too, as if the agent had almost forgotten as she was processing my card.

Didn't have to pay it last time I upgraded.

Bit annoying, but I guess they processed it as an Avios redeposit which does attract this fee.