GamePass Ultimate Existing Subscriber Question

The offer is for new subscribers only, not continuation of an existing one. I only know this because I tried to myself.

Hello TuPaco Grande,

Thank you for posting on the Xbox forums. We understand that you are interested in learning if you can cancel your subscription in order to apply a prepaid code for Xbox Game Pass Ultimate. If you have a prepaid code for Xbox Game
Pass Ultimate, you can definitely
turn off auto renewand
redeem the code when your subscription runs out. If you are purchasing a
promotion from the Xbox website, the system will detect that your account has had Xbox Game Pass Ultimate. Therefore, you will not receive the promotional offer. We hope this information helps TuPaco Grande!

The offer is valid only for new subscribers, but you can be attentive to the microsoft store, sometimes discounts on membership.

Were you aware of the promotion for existing gold/gamepass subscribers when converting to Ultimate? That is a good deal e.g. if you had 24 months of gold remaining you would get 24 months of Ultimate.

This is a promotion for new subscribers of the ultimate gamepass.