I just had a pretty bad experience with BAEC, and I wanted to ask here if there's anything I can do to avoid it in the future. It went like this:

Call #1: After realizing the book-and-upgrade form at ba.com does not support open-jaws, I try to make the whole transaction (book WTP with money and then upgrade to CW with miles) over the phone. I tell them exactly what I want (LGW-MCO out and JFK-LHR-OSL in) and am told cheerfully that I need to book the WTP ticket first, and that I can do so online. I'm nervous about doing it in two separate transactions as I don't want to be stuck with non-upgradeable WTP tickets, so I ask several times if he can guarantee that this will be possible, and he even checks availability for each of the three flights and assures me there's plenty.

Call #2: After having done as instructed in #1 and made the open-jaw booking online, I now attempt to make the upgrade with miles over the phone. Turns out this is not possible because the itinerary starts and ends in different zones. I believe this is correct, I'm just baffled that the person handling call #1 didn't tell me, considering how carefully I asked for reassurances that what I was trying to do would be possible. All things considered, I was satisfied with the outcome - they voided the booking with no extra fee and told me to just book the LHR-OSL flight separately.

Call #3: After having waited 5 days (!) for the block on my credit card from the first booking to lift so I could book again, I've made the LGW-MCO/JFK-LHR booking online and am trying to upgrade it over the phone. After a long wait I'm told that sure, this is possible, but I have to pay a "change fee" in addition to extra taxes. I ended up paying in the end, mainly because I was starting to get worried that my booking was ticketed in the UK and thus not technically supposed to be upgradeable as that's not my membership country.

So I guess my question is: Assuming this "change fee" is legit given the sequence of events described above, is there anything I can do differently to avoid such a fee?

Also, fee notwithstanding, is there any way for me to skip the ba.com experience altogether and have them do both the booking and the MFU in one transaction during a single phone call? In this case there seemed to be no danger of the award availability disappearing overnight, but I'd hate to have to do this in two separate transactions if availability were scarce.