NAPIT did a FAQ video, accessible from their installer portal (where you do the notifications) addressing this scenario, and their view is that if you're doing additions and alterations to a circuit, and where the new Regs would require a Type A RCD (i.e. unless it's a fixed load that could not leak DC under fault conditions) then a Type A RCD is required, i.e. if there is an existing Type AC RCD then this is not sufficient, and you need to do something about it.
A lot of my work involves minor works - additions and alterations to existing circuits.
A lot of the consumer units I come across were manufactuered by the likes of GET, Vytec, Steeple, GE, F&G, Contactum (the old ones with the brown MCB levers), MEM, CED, Legrand, "Plug-in-Systems", Tenby; or old Wylex/Crabtree boards with BS4293 RCDs and BS3871 MCBs. Chances of getting an RCBO or even a Type A RCCB to fit in those boards is pretty much nil (with the possible exception of the MEM RCBO pods). With a few notable exeptions, getting information from manufacturers about whether they approve the fitting of current (Type A RCBO/RCD) devices into older boards is like pulling teeth. Sometimes they're happy to talk to you on the phone, but they're rarely - if ever - willing to put anything in writing, and writen enquiries are ignored.
So if a new board is not wanted/economically unfeasible, then it's either a Type A RCD spur if it's lights, or I guess an external metal enclosure with a Type A RCCB if it's sockets (like doing minor works on a circuit off an old BS3036 fuse board).
It's a lot harder to explain to clients why it's necessary, too. When RCDs in general were introduced, and became required in more and more circumstances, it was easier to point out the difference in how an RCD will protect you where a fuse or circuit breaker won't. I suppose you could go along the lines of, "Well, it's not that the old RCDs are not safe any more, it's more that the things we plug into the circuits have changed," route, but then they point out that LED lights, and power supplies etc have been around for many years so why didn't they do this sooner, it feels a bit like the sort of uncomfortable conversation you'd have with someone in the 1990's who'd bought a garage with an asbestos roof a few years earlier.
One of the things that the CPSes could actually do that's useful is to collectively go round all the manufacturers of all the devices, and get written documentation about what is compatible with what. Their advice to "contact the manufacturer to confirm compatibility" is all very well but my own experience is enquiries go unanswered; and there must be so much duplicated waste of time when everyone's asking the same questions.
