Xbox Game Pass Ultimate Upgrade - still converts 36 months?

Yes. The upgrade is not tied to a promotion. Anytime you have regular Gold/Game Pass and buy Ultimate, it will convert at a ratio of 1:1 for up to 36 months.

If you have 12 months of Xbox gold, and you buy one month of game pass ultimate. It will convert the remaining time to game pass ultimate up to 36 months total. So you would have 13 months of Game pass ultimate. It cannot be converted back

I'm assuming you're talking about the deal to convert Xbox Live Gold and Gamepass to the Ultimate subscription. If you read the description on this page
it says "Joining Xbox Game Pass Ultimate will upgrade your remaining Xbox Live Gold or Xbox Game Pass for Console or Xbox Game Pass for PC time into Xbox Game Pass Ultimate for the same amount of time, up to 36 months". So you will still be able to take advantage
of the deal. At the moment there's also an offer to get 2 months of Gamepass Ultimate for $2 on top of whatever is converted.

Yes. If you have a regular Gold or Game Pass subscription on your account and then purchase Ultimate, your remaining time will be upgraded to Ultimate. You can convert up to 36 months. So if you add 12 months of Gold, then buy the 1 month of Ultimate,
you'll have 13 months of Ultimate.

If it's just Game Pass then at the end of the 2 weeks Game Pass will end and you'll just have your Gold subscription.

If it's Game Pass Ultimate it will convert the gold you have until Ultimate as well. If the message you're getting says "Joining Xbox Game Pass Ultimate will upgrade your remaining Xbox Live Gold or Xbox Game Pass membership(s)
into Xbox Game Pass Ultimate for the same amount of time, up to 36 months total. All conversions are final; conversion ratio subject to change." Then the 2 months of gold will upgrade so you will have over 2 months of Game Pass Ultimate.