I've just done this too, and for those with a bunch of prepaid months on Gold and Game Pass this is a REALLY good deal.
For every month of Gold you have, you get a one month of Ultimate, and for every month of Game Pass, you also get a month of Ultimate.
I had Gold paid up to early January, and Game Pass until around late March, but following the upgrade (and paying £1 for the first month of Ultimate) I had Game Pass Ultimate running up to the end of October 2020! For a quid!
Amazon also have a deal on right now for 3 months of Ultimate for £14.99 (£5 a month - over half price), so I bought that, added it on to my subscription, and I'm now stacked with Game Pass Ultimate until the end of January 2021.
Here's the Amazon offer: https://www.amazon.co.uk/Offer-Ultimate ... %2C135&sr=
Not bad at all. I'd urge all current Gold and/or Game Pass to upgrade - it's a no-brainer....