Yes, but surely the minute you touch an existing installation, you start to become responsible for it?
If I have BS 3036 fuses and rubber cable in poor condition and then I replace my electric cooker with one of similar current draw to the existing one, I would not be changing an existing installation either, but can I legally do so? I know you can't do it the other way around (i.e. put a new circuit in to feed a piece of electrical equipment that is only running on Grandfathers' rights, and also in poor state), but can you do it this way round? My feeling is that, like running a new light fitting onto a circuit that would not pass an EICR, the answer is 'no'.
Having said all that, I would agree that most electricians would probably not interpret the regulations to mean a new boiler MUST be on an independent circuit, as Vee has not denied.