BA has an RTW dept in the UK
[email protected] but as anabolism wrote there may be issues in trying to make changes to the ex-SEZ tickets.
That's the one I've used. I had similar concerns re my ex-CAI booking. In that case I waited till I had flown the first sector but I guess if BA decided it was never a valid fare rather than no longer a valid fare you may want to be extra cautious.