i did it last week, on my xbox i installed "xbox insider" from the store then ran it and enrolled in Delta program (less often firmware updates). That took about 20 minutes to download and update the OS.
After that went right from the dashboard to Game Pass tab, enrolled in Game Pass Ultimate for $1, got an error message, but looking at my subs it successfully combined my GWG (gold) and Game Pass which each had ~1-1.5 years and now my Ultimate expires in Jan 2022. The max you can have is 3 years/36 months or May 2022, not sure what happens if you have more than 36 months combined.
Its totally worth doing for anyone, because it'll charge you $1/mo for ultimate right now, even better if you already have gold/gamepass since it'll roll it all up into one sub.
It is beta but the latest update fixed most of the bugs I think, before people weren't able to go online, download GWG games, etc but all that seems fixed for now. I think the only maybe bugs are that you can't add gold/GP sub codes to your account for now, but since you are being charged only $1/mo for it its better to hold those codes I'd imagine anyways. If you do have some codes to redeem, I'd redeem them first THEN enroll in ultimate so it combines it all up.