Gold to Ultimate
Upgrading from Gold to Ultimate is 1:1
Without a Gamepass subscription it's 1:1, but while it's active it's not.
https://www.xbox.com/en-US/live/gold/upgrade
Once you have Ultimate, any Gold you try to add to an active Ultimate subscription does not convert 1:1. So you would want to hold onto that Gold until after your Ultimate subscription ends if you want to get the full value. At that point then you would do the 3 year purchase+convert again. Unless everything changes between now and then.