I am confident in saying I do not think this action would pass the "reasonableness" test and the association would be ruled against in a court of law or by arbitration. They either need to assign all units parking spots or none at all. While they can change the location of the assigned spots, taking them away all together is unreasonable.
The issue with your condo having an extra paid parking spot is a different matter. The payment should have triggered a recording of some sort of deed. Without any paperwork you have no evidence to prove your claim. Search the county's official records online database by the original owner's name to see if you can find it.
You should send a certified letter to the condo telling them of your dispute, your requested resolution and that you may exercise your right to arbitrate the matter per Fla. Stat. 718.1255 if not resolved.
This communication is not intended to create an attorney/client relationship. It is always recommended you consult an attorney in person to discuss your case. The Law Offices of Stage & Associates practices state-wide and represents homeowners and community associations. Please visit our website at www.stagelaw.com.
It certainly does not appear that the Association is acting correctly. I encourage you to seek the counsel of an attorney familiar with HOA and Condo Associations to fight on your behalf.