Hey there digiden,
You should be able to simply install the PRO version and activate it and everything should be fine. You’ll want to be sure that you’re running the latest release of both the free and the PRO plugins though when you make the switch to minimize the potential for any unforeseen problems.
Have you had problems? Or are you just nervous about it? If just nervous, always remember to make a backup of your site before doing anything that causes you to be concerned. Or just backup because it’s a good idea most anytime!
If you need anything else, please ask!
— Julie