Is the fast resize availabilty something which can change in a datacenter in relation to the actual resources or a limit set in place by someone from the staff?

Eg. If I have 2 droplets in the London datacenter and I destroy one can I resize the other droplet in order to upgrade my plan?

I'm asking this because I'd like to upgrade my plan on a London droplet (where fast resize is not available) and I care about downtime (so I'd prefer skipping the snapshot method).
I'm considering waiting a few days to see if it becomes available again but I'm not sure if it's pointless

Thanks,
Federico