If you are trying to change a transit to a stopover, then that is something that you are not, I think, going to be able to fix before you get to London. iirc , changing a transit to a stopover counts as a routing change ( though I could be mistaken )

With the tickets that I organised, I had originally booked nearly all the sectors as having stopovers; when making the change to 2nd sector ex LHR, it was same destination to a later date where there already was a stopover allowed for in London