Existing members of Game pass ultimate

You would buy it the same as anyone else would, but you'd wait until you're down to 12 months left on your subscription before redeeming the 24 months of Ultimate as you can only have up to 36 months at a time.

Did a bit more digging in the FAQs, no issues at all for existing Game Pass members (direct quote from FAQ):

If you're already an Xbox Live Gold or Xbox Game Pass member, any days remaining in your membership(s) will be converted to Xbox Game Pass Ultimate at a conversion ratio when you redeem the 24-month Xbox Game Pass Ultimate
membership and added to your Ultimate membership up to a maximum of 36 months. Xbox Game Pass Ultimate memberships are limited to 36 months.

If you’re already an Xbox Game Pass Ultimate member, wait until your existing membership has a prepaid balance that is less than 12 months before redeeming the 24-month Xbox Game Pass Ultimate membership through Xbox All
Access. You will be unable to redeem with more than 12 prepaid months of Xbox Game Pass Ultimate.

To check your prepaid balance, go to https://account.microsoft.com/services. Note, once converted to Xbox Game Pass Ultimate, Xbox Live
Gold and other Game Pass memberships cannot be unconverted. Learn more at www.xbox.com/gamepass.

Hi,

Did you recently make the ultimate game pass purchase or did you already have the ultimate game pass?

If you made the purchase since the new deal was offered you should receive a notification or an email with a redeem code in about 10 business days. If you are an existing ultimate member the off does not apply.

You can add the 24 months of Ultimate to your existing subscription as long as it won't go over 36 months. If it were to go over the 36 months, you would need to wait until you have 12 months or less remaining on your subscription before redeeming the 24
months of Ultimate.

https://www.xbox.com/en-US/xbox-all-access#faq

An Xbox All Access purchase includes 24 months of Xbox Game Pass Ultimate. So, what happens if I have an existing Xbox Game Pass and/or Xbox Live Gold membership when I purchase a console with Xbox All Access?

If you're already an Xbox Live Gold or Xbox Game Pass member, any days remaining in your membership(s) will be converted to Xbox Game Pass Ultimate at a conversion ratio when you redeem the 24-month Xbox Game Pass Ultimate membership and added to your Xbox
Game Pass Ultimate membership up to a maximum of 36 months. Xbox Game Pass Ultimate memberships are limited to 36 months.

If you’re already an Xbox Game Pass Ultimate member, wait until your existing membership has a prepaid balance that is less than 12 months before redeeming the 24-month Ultimate membership through Xbox All Access. You will be unable to redeem with more than
12 prepaid months of Xbox Game Pass Ultimate.

It doesn't say previous customers, it says Not valid for existing Xbox Game Pass Ultimate members, other exclusions apply. Offer available in all Xbox markets excluding Russia.