I must admit I find English is a politician language, it is so easy to say some thing, and the reverse being perceived. You don't want that cake, is answered as yes which means you don't want it, however many think the are saying the reverse.
The BS 7671 seems to have small changes, traditionally and historically for example, one needs to be an English student to work out what it means.
Decimate I think of as being kill one in ten, derives from the Latin, when if a battle was lost, one in ten were killed to ensure they fought harder at the next battle. OK kill is dropped to get rid of, but even dictionaries now seem to think it means get rid of nine in ten. OK this is an extreme case.
However what does
Existing installations that have been installed in accordance with earlier editions of the Regulations may not comply with this edition in every respect. This does not necessarily mean that they are unsafe for continued use or require upgrading.
mean?
I take it to mean there is no requirement to update, and the regulations are not retrospective
Installations designed after 30st June 2008 are to comply with BS 7671:2008.
which has been repeated in every edition, so if it complied when installed, it still complies.
However I am sure most would agree knife switches with exposed live parts are no longer permitted. I remember using them on the Queensferry bridge (Wales not Scotland) and plunging the village into darkness as had overloaded the supply, the base of the bridge had concrete in, and was now too heavy to lift, I said nothing at the time, and no one seemed to work out all down to me. But point is around 1970 and still had knife switches. Shocking I know.
We still have the problem, how far back can we go? When I started the front loading video player was still not invented, so no switch mode power supplies. Today they are the norm, so now good reason why we need the SPD, however not having them does not result in danger, so desirable but not essential.
So with the required/desirable question, we are always going to have differences in option. I hope I have an open mind, and can see how as time goes on we need to change. But this will likely produce some debate, and so what may be considered as unhelpful posts, electrics is a complex subject, so debate is good.