Upgrading to game pass ultimate with less than a month of live remaining

I’m trying to get game pass ultimate now so its on a date that works for me but it wants to upgrade my remaining xbox live and i can’t cancel that it just says contact support, if i upgrade to game pass ultimate is it going to just give me ultimate for my
remaining live days or will it give me the full month? I dont want to be paying full price for half a month mostly because i want to have subscription roll roughly on this date not in a few weeks when my current sub ends.
So its 1st may now and my standard live memberships ends 12th, if i upgrade to ultimate will ultimate end on 12th or will it give me the full month?

Also theres not option to just purchase game pass seperately it automatically tries to upgrade, one solution of course would be buy non ultimate game pass but i still need live i just dont want pay date on the current date its on

:)