Over here we can get small plastic enclosures which will accept a DIN-rail RCD, with space for just one or two MCBs, often used in a garage or workshop these days.
I use such an enclosure with a new RCD (and the MCB spaces blanked off) to replace the older voltage-operated ELCBs. That way the main panel doesn't need to be replaced. (Unless there are other reasons for changing it, of course.)
But I agree with you that these new NZ regulations seem ridiculously over-the-top. Requiring an RCD on new installations, or on a major rewire is one thing, but specify that a perfectly serviceable and safe main panel has to be changed just because you've replaced a light switch is crazy.
Once this new rule becomes known to householders, I can see the situation developing where people with no electrical knowledge will be forced to "have a go" themselves simply because they're not going to (or cannot afford to) pay a sparky for what starts out as a simple 10-minute job and turns into a major panel change.
