I think you would probably see a problem if someone 'replaced' an RCD with a main switch, or a 6A MCB with a 32A one, wouldn't you?
Well, obviously there might be an
electrical problem with that (notwithstanding that the first one might be perfectly acceptable if there is no specific requirement for the RCD to be there). But from the purely
regulatory standpoint of schedule 4 of the building regulations, I think the exemption from notification in 1(a) could still be said to apply (although obviously replacing a 6A MCB with a 32A one if the cables were not suitable etc. would fall foul of the general Part P requirement of "reasonable provision" for safety).
Attempting to list all of the possibilities would obvioulsy be impractical and futile - maybe they could use some phrase like the "which doesn't (maybe 'adversely') alter the characteristics of the circuit", similar to that which they have used in other parts of the Schedule?
Which has to raise the question: If that is what was really intended, why was such a clause not included in 1(a) as it was in 1(c) & (d) ?
That aside though, at the practical level, one has to question how anyone who is not competent enough to be doing the work in the first place could be expected to
know whether his changes would affect (adversely or otherwise) the circuit protective measures, current-carrying capability of the cables, and so on.
In the case of 1(d), for example, if somebody understands enough to realize that his work of installing additional thermal insulation will affect the cable's current-carrying capacity, then surely he understands enough to do the work safely without it being notified anyway? Or, conversely, if he doesn't understand that the work planned will affect how much current the cable can carry safely, how would he realize that 1(d) requires that work to be notified?