My past experience in changing a booking was like this:
call UK centre to initiate changes. The agent will claim the change is made, but need to reprice manually. Usually this will led to dead end, no confirmation on new price, but if you head to MMB, all segments (new and old) will appear as a mess.

You then call India centre ask to complete the repricing, maybe 3-5 days after first call. This will take at least 15-30+ minutes waiting, but all manual repricing should complete in just 1 call. Mine case involves further changes and ended up taking 3 hours to complete,maybe shorter if things are going your way.