Shortly after the previous post here, I realised that the developer site was requiring two-factor authentication, not just the two-step authentication I was waiting for for the new ID I’d set up using an email address at one of my own domains. This meant that, barring setting up a new login on my laptop solely for the purposes of logging in to iCloud and/or the iTunes and App stores with the new ID then never using the login account again, I would need to use either my iCloud ID, or my Store ID as my developer ID. I realised that while I didn’t want to change the former from being my primary email address, I’d already changed the latter once when I was required to change it from being a non-email address 7-letter string. Unfortunately, having used the email already set up at my domain (the domain I’d already been using as the basis for bundle identifiers in Xcode), I couldn’t change the existing ID to that email address, even after changing the new ID to a different email address. So, I set up another email address at my domain (this time using the same 7-letter string before the domain name that had been my old developer id, and was before the @ sign for my apple hosted email), changed the current developer and Store ID from the gmail address to the new address, and renewed the apple developer program membership (under the new new name of the old ID) rather than signing up for a new apple developer program membership.

I then downloaded and tried out the Xcode beta, decided I needed the Catalina beta as well, added a partition to my laptop, installed the Catalina beta on the new partition, installed the previously downloaded Xcode beta on the new partition instead and removed it from the current partition. All seems to be working as it should.