How can I reassign the nominated organiser / owner user for my Microsoft 365 Family subscription to be one of the other already-existing family members?

Question

I have a Microsoft 365 Family subscription which is currently owned by a user account, let's call it "User A", which has Administrator access rights on my Windows PCs so that I can install and maintain software using an account that is not in normal daily use. I also have other regular user accounts as members of my family subscription which do not have admin access rights on these PCs. I keep these accounts separate from the Administrator account, "User A", for the obvious security reasons. My enquiry is about how I can transfer access to the advanced features of my Microsoft 365 Family subscription from "User A" to one of the regular users mentioned above, let's call it "User M", so that this user gets access to the advanced features of Microsoft 365, such as AI integration with the Office apps (especially from within the desktop ones). Can someone please briefly explain to me step by step what I need to do to move the subscription's primary user email address from one family member to another? I tried using Copilot to ask this question and it guided me to the process to change the primary email address and roles by adding User M's email as an alias and make this the primary alias, but it looks like Microsoft may have updated its account settings interface and the option to "Manage how you sign in to Microsoft" is no longer available in the "Your Info" section of the "Microsoft Account" management pages. I eventually found a "Manage how you sign in to Microsoft" page on account.live.com and clicked on "Add email" in the "Account alias" section. When I select the "Add an existing email address as a Microsoft account alias" option, enter User M's email address and click the "Add Alias" button, I get an error message that says "This email address is already taken. Please try another."

Just to clarify my question: I do not want to share the advanced (AI) benefits with additional family members, I want to change the nominated family account organiser / owner to be one of the other already-existing family members so this other member (and only this one) can use the AI tools, etc.