"This code is not currently supported for conversion to your Xbox Game Pass Ultimate...

Hello Andre,

Thanks for posting here on the Xbox forums. You can find this information in the Xbox Gamepass ultimate FAQ. ill go ahead and quote the part that is relevant to your question :

With Xbox Game Pass Ultimate, can I still redeem codes for Xbox Live Gold and Xbox Game Pass?

Yes. These codes will automatically convert to Xbox Game Pass Ultimate at a ratio based on the ERP (Estimated Retail Price) of those codes. Conversion rates are subject to change.

The conversion rates are currently set to "12 month gold pre paid card = 4 months Game Pass Ultimate. You can find the conversion table at the bottom of this page here Xbox Game
Pass. Just select "With Xbox Game Pass Ultimate, can I still redeem codes for Xbox Live Gold and Xbox Game Pass?"
to see the rest of the conversion rates.

Hope this helps!

No. As stated before you upgrade, once you're current subscription is converted, there is no way to go back.

Xbox Game Pass Ultimate:

Use your active Xbox Game Pass Ultimate membership to play games on Xbox One and Windows 10 PC. Xbox Game Pass Ultimate includes Xbox Game Pass for Console, Xbox Game Pass for PC, Xbox Live Gold and additional benefits. 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).

If you already have Game Pass Ultimate, the Game Pass will be converted as Ultimate as well at a different conversion rate. If you want only the Game Pass, you'll need to either cancel your current subscription or wait for it to end before you add the
new code.

Hi Noam

Yes it will add to your current game pass Ultimate.

The conversion ratio will only apply for Xbox live gold and game pass code if you have an active game pass Ultimate.

You may refer here: https://support.xbox.com/help/subscriptions-bil...

There's a conversion rate when you swap from XBL Gold or Game Pass to Game Pass Ultimate, there used to be a limited time deal where it would convert it at a 1:1 ratio when Game Pass Ultimate first launched but I don't think that applies anymore. Before
you confirm the conversion to Game Pass Ultimate, it usually tells you how many months your current Xbox Live Gold subscription will convert to.

If you already have Game Pass Ultimate and want to use an Xbox Live Gold code then I think 1 month of Gold or Game Pass gives you 20 days of Game Pass Ultimate.

There's an FAQ at the following link, if you click on the question that says "After I join Xbox Game Pass Ultimate, can I still redeem Xbox Live Gold and Xbox Game Pass codes?" there will be a full conversion table: https://www.xbox.com/en-US/live/gold/upgrade