1 month ultimate I can't use it so

The 12 months of gold will be upgraded to Ultimate so you will have 12 months plus the amount of Ultimate you purchase (I can't remember if it's 1 month or 3 months), so if it is 1 month, you would have 13 months total of Ultimate. After that, you won't
have to pay the £10.99 a month until the 13 months end.

The offer of £1 is purely for new members to the GamePass ultimate subscription.

There is a way to get ultimate at the cost of gold plus one month of ultimate but you will have to wait now without owning GamePass for the required time (i believe its 1 year but can't confirm that)

It is a straight conversion of 1:1

I ended up with 36 months of Ultimate for the price of Gold membership for the three years (I got it on an awesome offer of £24 a year) and then paid the £1 for 3 months of GamePass Ultimate. 36 months is the limit to how many months it will work for so
there was some overspill but essentially it cost me £73 for three years of GamePass ultimate

Here is the breakdown of how conversion works. We will use 1 months ultimate as an example.

If you have ultimate active for 1 month for example:

If you add 12 months of gold while ultimate is active, the conversion is:

12 months gold = 4 months ultimate

so 4 months ultimate + 1 prior month ultimate = 5 months ultimate

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If you have only 12 months gold active:

Then if you add 1 month ultimate, the conversion is:

12 months gold = 12 months ultimate

12 months ultimate + 1 month ultimate = 13 months ultimate.

Do you understand? If you want a better conversion rate, then you need gold first and then add 1 month ultimate to make everything else ultimate but if you add gold later, it gives you less months.

Yes. If you use the 12 month first and then purchase the $1 promotion, it will upgrade your gold to Ultimate. So you will get 13 months of Ultimate for $1 basically.

If you upgrade to Ultimate using the $1 promotion, everything you already have will be converted to Ultimate. So you'll pay for 1 month but get 4-5 months of Ultimate before needing to pay again.