That's obviously the sensible course, hence 'how it should be', but I have to say that our experiences appear to differ. Over the years/decades, I've known quite a lot of people who have moved from E7 to a single-rate tariff (usually when they have got rid of storage heaters) and in virtually all cases the supplier has reacted by (totally unnecessarily) replacing their dual-rate meter with a single rate one.AFAIK that's been standard practice for years (for ever?) ... they know the meter has two registers, so they they take both readings, calculate both usages, and add them together to find your total usage.
Mind you, 'smart' meters have changed all this. Relatively 'dumb' though they seem to be, the one thing they do offer is multiple time-dependent metering registers, so the issue of what tariff one is paying for becomes rather moot.
Kind Regards, John