Never stop this type of medication cold turkey, it's dangerous. You'll have to see your doctor to get back on Celexa let him/her guide you on this. It all depends on the medication when it comes to switching from one to another. Some you can start the day after your last dose of a previous medication, others need to be gradually introduced into your system as you gradually decrease the old one. Do this under the care of your doctor.
Just to be on the safe side, you may want to take a few days to wash out that Prozac before introducing the new SSRI.
Some doctors will just advise one to make the switch with no wash out, but I always like to at least wash out some of the old medication before starting a new one.