Seems like Apple implemented some strange solution to merge me.com accounts with iCloud environment. Like keeping two independent accounts in backend and merging it artificially as one in frontend (GUI)? Just improvising.I had a negative experience with this. Back from 2011, my AppleID/AppleAccount was a @gmx.de mail address. Now I tried to move to my @me.com mail address as primary e-mail. First, all seemed well - until I noticed, that the AppleTV+ subscription was gone. Although in settings (iPhone, iPad) the subscription showed up as active, my AppleTV 4k nevertheless handled me as a new customer. Had to revert to my original primary e-mail for my AppleID. D´oh!
Changing primary address from/to external to/from icloud.com doesn't affect subscriptions (at least in my case).