Xbox live gold to game pass ultimate

Ok so i am wondering about converting my 9 months of xbox live gold to games pass ultimate. I see the option to pay £1 for the first month and then it is £11.99 a month after that. If i do this what will happen to my remaining xbox live gold months. It
is my understanding that i will pay no extra apart from that £1 for the next 9 months until my subscription runs out. Is this correct? I am scared of losing those 9 months and having to pay £11.99 a month from there

:)