Game pass conversion

It will extend your Ultimate, but at a conversion rate. You can view the conversion rate in the

Game Pass FAQ.

It would convert to Ultimate but at a ratio of 3:1. 12 months of Gold would convert to 4 months of Ultimate.

You would pay once up front and then you wouldn't pay again until your renewal date.

Note: Purchase of Xbox Game Pass Ultimate automatically upgrades your existing Xbox Live Gold or Xbox Game Pass membership(s) to Xbox Game Pass Ultimate at a conversion ratio each based on days remaining.
Learn how this works at www.xbox.com/gamepass. Future code redemptions are also subject to a conversion ratio. Conversion ratio subject to change. All conversions to Xbox Game Pass Ultimate are final: Once converted, you cannot re-convert your Xbox Game Pass
Ultimate membership back to a previous membership. Maximum 36 months of redeemed Xbox Game Pass Ultimate per account at a time, including any converted time from Xbox Live Gold and/or Xbox Game Pass (for PC or console).

Some Day when the Game Pass Ultimante was Launched you make the conversion the your Old Game Pass to Game Pass Ultimate, If you done, all Live Gold or Game Pass you own go be converted to GPU I Think, or you need all your game pass subscriptions over,
or make the conversion of the all new Game Pass you go have