As sparky3366 observed if the upstairs and downstairs circuits had a borrowed neutral then it is possible that when the board was fitted the installer chosen to join the 2 together.
In the borrowed neutral scenario if the 2 circuits been put on separate RCDs then the RCDs would have kept tripping, as current from one circuit would be going down neutral of other hence imbalance and trip. Had they been on separate MCBs on the same RCD then safe isolation would be an issue under 314.4 and 314.1 vi which is in my view a more important reg (as it could result in a shock for the next person to work on the circuit)
Having said that there is a choice and that is to remove the borrowed neutral, so that the circuits are truly seperate, this is by far the most desirable solution although it may well involve pulling the house apart!
It would have been nice to get an explanation as to why the course of action was taken but it may transpire that it was the lesser of a number of evils! The customer should have, in my mind, had these options explained before action was taken and the "deviation" noted and explained on the installation certificate for the new board ( so as to avoid confusion for the next spark)
Now if there is no borrowed neutral, and they were separate it just seems wrong to me