No legal requirement to change it per se, I've seen these adverts who offer a "free electrical safety check" if you have a non-MCB board, and I presume after that it's all hard sell to upgrade it to people who will probably not know what they're talking about. For instance how many times have I been asked "is this safe" about a bit of bonding under the sink or in the bathroom.
Whilst no-one would dispute RCDs are a good thing, I sometimes wonder about old people, and if a lightbulb blows it can take out the lighting. OK if the lighting circuits are spread accross two RCDs taking lighting arrangements into account, but if not they are left with no lights.
