I think if you have an open ring that is one or two of the 3 conductors it should and can be fixed fairly easily, so I would not accept a MCB change as a fix.With regard to switching out a 32a mcb for a 20a, how do you all feel about a live conductor potentially floating around in a back box somewhere if this has been done because there is open circuit rather than multiple sockets from same spur off ring
However, if someone has completely opened the ring in to two radials, or has added multiple spurs, you might find that a proper fix is difficult = time consuming = expensive. In this case a MCB change is acceptable in my mind.