It is my understanding that if you buy any upgrade and cancel gamepass, you will need to buy the standard version to access the upgrade.
The idea is that Microsoft doesn’t want you to cancel Gamepass, so if you do you will need to pay $20-25 more for the premium/deluxe premium than you would if you bought it without gamepass as a “punishment” for leaving.
It’s explained in this thread:
The wording is also extremely misleading in the description of the product you are about to buy in the in game marketplace:
“The premium deluxe edition includes everything from Microsoft Flight Simulator plus 10 additional highly accurate planes”
Microsoft really needs to make clear that you need to purchase the standard edition to use any upgrades you purchased for the Gamepass version. This is really cynical stuff.