241 and GU2 voucher

Hi all, first time using the 241 and GU2 voucher. I thought I understood the rules but seems like I am at a dead end. Really appreciate your thoughts.

Booked biz class LHR to IAD for 2 people with 241 (160K Avios and 750 GBP tax), then called BA to upgrade to first with GU2. Spent over 1 hour with the agent, who seemed pretty clueless at first, finally she said “just add roughly 500 GBP more in taxes.” Fine. 4 days later still no e-ticket, though the app is still showing seats in First. Phoned BA, new agent said “the previous agent was wrong, disregard it. It should be an additional 1800 GBP with a refund of 60K Avios,” saying something about from biz class to first class, it’s no longer a flat fee, but with higher cabin tax, which I don’t fully understand. Anyway the total cost becomes “100K Avios and 2550 GBP”, which seemed a little steep. What do you guys think? Who’s right/ wrong? Thanks so much!