I did an UuA earlier this year to CW from WT+ (On a different route). They originally said that there was no difference in taxes and fees etc to upgrade so they just took the Avios. I then got a call 30 mins later saying “sorry we made a mistake and there actually are fees, it is €xx in total for the two of you” I can’t remember exactly how much but it was along the lines of £55 when converted from EUR.

it seems that there was no change fee incurred for me. I don’t know if this is always the cause when UuA?