You can reserve any number of IP's, in any number of data centers by clicking that link -- this will allow you to reserve a Floating IP first, then you can go in, remove the existing from the Droplet, replace it with the new Floating IP and finally, remove the old Floating IP from your account.
As of right now, there's not a way to be selective without using the above method and having two IP's for a limited amount of time (until you remove the old one).
DigitalOcean's backend is setup to attempt to give a customer the same IP that they had if they IP has not yet been reassigned in the time it took to reattach it to the account. If you remove the floating IP and attempt to grab another one seconds later, this is most likely what's happening.
The above solution works around that, though it does introduce one more step to the process :-).