It's not even as complicated as suggested above.
Everyone on GPON gets the full 2.4Gb/s down and 1.2Gb/s up to their ONT.
Only the upstream has shared time slots and that has nothing to do with package speeds.
A user on a higher service does not get a bigger share of time slots.
It's only when a user is physically uploading that time slots are assigned.
It's simply a profile applied to a line, artificially limiting throughput. No more, no less.
If you want to change speeds you tell the ISP, they order that change from OpenReach, who then push a button changing your profile.
It can be a same/next day change.