Transfer Rules re Portfolio ISAs

I currently have a portfolio ISA with Nationwide and am therefore able to pay into both a Help to Buy ISA and standard easy-access ISA, and have done for the last few years.

I like having both and intend to max (i.e. 12k) the HTB as a priority, but the rate on the easy access account isn't ideal.

I don't fully understand the rules surrounding transfers, but from what I can gather, I could transfer my previous years' contributions from the easy-access account to a new provider and leave it there, whilst still keeping my newest/current contributions in the Nationwide account and continue paying new money into it?

The only issue would be if my HTB contributions are transferred too, as I'm not sure if the portfolio distinction would be made when requesting the transfer. I'd be interested to know if anyone with a portfolio ISA has actually managed to transfer part/all of it (or split it) successfully as it was one of those things that sounded like a good idea at the time but now seems more trouble than it's worth!