Ok.
This what I found.
At the moment I am D/Loading MSFS Premium Deluxe. (PD)
In getting there I could not find a way to stop MS linking my Xbox Game Pass discount to the PD purchase. I logged out of the a/c, tried other things, but to no avail.
In the end I deleted the Xbox App & had to open a new Microsoft Account to get through the purchase process at full price.
So I currently have MS A/C ‘A’ which is linked to the still live, but uninstalled, Game Pass Standard MSFS, & MS A/C ‘B’, which is hooked into PD.
When I did the initial PD download it booted ok, then up came a "Press Any Screen to Continue’ message. Pressing a key did nothing.
I researched the issue & came across someone who had the same issue & fixed it by reinstalling the XBox App on their PC. I went that route & after also installing XBox Identifier, the next PD boot went right through to the D/Load 127 GB screen, to complete the installation.
As KodiakJac indicated, Win10, the Microsoft Store, your Microsoft A/C, The Xbox App & MSFS are all inextricably tied into each other.
Hopefully PD will run ok.
What I am waiting to see now is if a couple of Carenado aircraft I purchased via MS A/C ‘A’ can be made to install in PD!
TG
Postscript: The short answer is that the aircraft can’t be transferred from A/C A to A/C B.
However XBox support suggested I could apply for a refund from A/C A & then repurchase fo A/C B.
In the meantime I found that I could copy the aircraft out of the Standard MSFS & drop them in Premium Deluxe Community folder, & they work!.