I suspect they need to be rebooked into the same fare basis (ie. U for MFU redemption, T for discounted WT+, etc) and perhaps that fare basis is not available on the day you wish to travel.
Also what BAAZ has said above could be relevant.

Admittedly I find it hard that it was agreed not to charge you in the future, especially as BA were not at fault in any way for the circumstances. I would've thought they'd put this in writing though I could well be wrong.

Honestly I've never heard of this being allowed, particularly when not confirmed in writing, but others could have a different experience.

As such, I suspect you may have to pay what is now due.