Managed to cancel proposed timeslot and arrange new second BioNTech dose slot online just now.
No problems.
Just rebooked mine as well, easy.
Instead of asking if there are enough vaccines on order, I think the question is, is there enough people to get vaccinated?
https://booking.covidvaccine.gov.hk/centre/index.html
Booked mine just now ( BioNtech German sourced ). Loads of options so chose next Tuesday after lunch.
Also re-scheduled mine this morning, got the same day as was assigned by the govt for my re-booking, but switched it from Ap lei Chau to SYS.