Can't upgrade from gold to ultimate game pass

When you upgrade from standard Gold/Game Pass to Game Pass Ultimate, you upgrade at a ratio of 1:1.

The 3:1 conversion ratio is what happens when you already have Ultimate and are adding standard Gold/Game Pass to Ultimate.

Yes it will. If you are upgrading from a standard gold or game pass to Ultimate, it will upgrade at a ratio of 1:1. The FAQ does mention a different conversion ratio, but that is if you currently have Ultimate and are adding a standard gold or game pass.

That's saying that whatever standard Gold or Game Pass you have on your account will be upgraded to Ultimate when you purchase a new Ultimate subscription. Once you've upgraded that subscription, it can't be reverted back to just Gold/Game Pass (so if you
upgrade to the 15 months and cancel, you lose everything). The conversion ratio for upgrading to Ultimate from Gold/Game Pass is 1:1 (12 months + 3 months in your case). The conversion ratio if you have an active Ultimate subscription and are adding Gold/Game
Pass is different and based on price rather than time (this is not what you are doing currently).

There will also be a pop-up when you add a prepaid code to tell you what's going to happen when you redeem the code.

That means your gold was upgraded to Ultimate (Ultimate includes Gold, Game Pass for Xbox, and Game Pass for PC)

the upgrade from Xbox Game Pass to Game Pass Ultimate combines Game Pass with Xbox Live Gold, that is why Game Pass Ultimate costs more than standard Game Pass, and the combined cost is cheaper than having Live Gold and Game Pass as separate subscriptions