Ultimate codes

If you have just Gold currently and are adding the Ultimate code, the Gold will be upgraded to Ultimate and you'll actually have 15 months of Ultimate.

The code is only added as 4 months of Ultimate if you have Ultimate and are adding a 12 month Gold or Game Pass to that. Not the opposite.

If you are already a Game Pass Ultimate subscriber, then any Gold codes you redeem will be converted to Game Pass Ultimate. If you are not an Xbox Game Pass Ultimate Subscriber, then no, the Gold codes should not convert to Game Pass Ultimate.

It is an ultimate code. So just to be sure the code is for 2 weeks but it will turn my 2 months of gold into 2 months of ultimate?

Gold and Game Pass (not Ultimate) can be available as 14 day trial cards and would not be the same as Ultimate. If you're buying a trial code and you're wanting Ultimate, you'll want to make sure it is listed as Ultimate. If you buy a code that is only
for Gold or only for Game Pass, you will not have all of the features of Ultimate.

It will convert that way if you have a Game Pass Ultimate subscription. Since Ultimate includes both Gold and Game Pass, if you add a standard prepaid code, it will convert the code to Ultimate, but at a rate based on price rather than time (12 months
of Gold is equal to 4 months of Ultimate). In order to get the full 12 months of just Gold, you would need either cancel Ultimate or wait for it to run out before adding a code. It does warn you of this when you add the code.