Sorry for my bad english. I meant that once the full version of this Custom Rom is installed, then future updates, always on this rom, can be easily done without having to delete or format the device every time.What do you mean?
Updating the same ROM can try dirty flash, but always keep in mind there's chance to fail. Ask the developer if the update can be dirty flashed.
Switching between different ROMs, you have to format data. Or disable data encryption, with the downside of bad privacy and security.