Due to the lengthy programme of renovations at BP, any potential move to BP for C&C is at least a decade away, irrespective of when Charles becomes King.
There is certainly an argument for W&C to remain at KP in the next reign, though I am unclear of what will come of CH in the period between C&C moving to BP (at least a decade away), and William becoming King. It is far to substantial and grand a residence to be suitable for Harry.
As regards KP, I think the next 10-15 years will probably see a reasonable amount of change, as the Gloucesters and the two Kent couples pass on. How the freed up real estate gets used remains to be seen.
I wouldn't be surprised if it's Charles' intention that CH is just kept in waiting for Camilla for the possibility that Charles, after his ascension, predeceases her.
First, if Charles predeceases her after their ascension, Camilla will be a widowed Queen Consort (yes, yes, Princess Consort, but legally speaking, she'll be the QC), and you can't just pack off a widowed QC to
Ray Mill House. This isn't 1547.
Second, even beside the optics, a hypothetical widowed QC/PC Camilla will have work she wants to do and need a London base to do it from, much like the QM did. She's not going to drop all of those patronages, and she seems genuinely dedicated to her work with literacy and abuse victims. Also, I see her as continuing to host the Christmas party for ill children for as long as she's physically able.
Finally, I can see Charles seeing it as the house of his beloved Granny being the widow's residence of his beloved wife, given how much he loved/s both women.
Given that we've been told that W&K will stay at KP until such time as they may be called to serve as king and queen, and that we've been told Harry and Meghan will make their main home at Frogmore, I think it's most sensible to take things at face value and not expect to see H&M in a large apartment in London.