Xbox live ultimate help

Hi everyone, I can't get a clear answer on coverting my xbox live gold membership into ultimate. I took advantage of the £1 introductory offer last year which converted my remaining xbox live gold into ultimate. Now, I let that run out with auto renew
turned off. I have now built up 3 years of gold before microsoft rid the 12 month subscriptions and I now want to convert that back into ultimate. I have gone to the ultimate subscription page which says 'Joining ultimate will convert your remaining xbox live
gold for the same amount of time up to 36 months', but the next page (payment page) it says starting today you will be billed £10.99 per month. Basically, will microsoft bill me monthly (from today) or will they not bill me until 2023 when my membership is
set to expire? Very confused with this and any help would be appreciated before I go ahead with ultimate.

:)