Games Purchased during Gamepass Ultimate

Yes you can because you bought the license from the store so when you game pass expires you will still have it

If you download a game through game pass then you will not have the game anymore when the subscription expires. If you purchase a game through the xbox store then you will keep the game even if your game pass subscription expires.

https://beta.support.xbox.com/help/subscriptions-billing/manage-subscriptions/xbox-game-pass-faq

"Games you install using Xbox Game Pass remain playable once they’ve left the Game Pass catalog only when you buy them—and you get a hefty discount when you buy games as an Xbox Game Pass subscriber.

When you buy games at a discount, those games are yours to keep. Similarly, any add-ons you buy are yours to keep, whether you buy the base game or not.

Either way, whenever you sign in to Xbox Live, your game data gets saved online. That means, whenever you next play the game—if you buy it, if you play it at someone else’s house, or if it comes back
to Game Pass—you’ll be able to pick up right where you left off."

Hi there DimitriFR

You can upgrade gold or gamepass to ultimate which which will do it but during this post time frame there is issues on the purchase side of things.

Are you trying to purchase the current promotion for Xbox GamePass Ultimate?

If so, the current promotion is only for new members/subscribers to GamePass Ultimate. If you had GamePass Ultimate before you will not be able to purchase it.

Hello!

To convert your existing Gold plan, you need to purchase any amount of months for a Ultimate subscription. Once you do, your current plans (Gamepass and/or Gold for instance) get automatically converted to Ultimate. A message is also displayed during your
purchase for your Ultimate subscription, notifying you about the conversion and asking you for confirmation.

Hope I helped!