Yes, I've had the same experience before. I had LHR-HEL as part of a DONE4 and was trying to change the flight from BA code to AY code (it was AY metal), the surcharges for AY code would have been significantly (>£100) higher than those on BA code so I didn't do it. It's also likely that the ticket would need to be re-issued which would trigger the change fee of $125.