I'm afraid I don't share Saildude's view about being bent out of shape - sorry Saildude. If I paid for a plugin which was subseqently distributed via the repository, I'd be pleased to have helped it's development and I'd hope that others would feel like making an equivalent donation if they find it useful.
If the licence is GPL (or GPL compatible) then it isn't a problem. The people who have previously purchased the plugin shouldn't mind that more people wil be using it should they?
Even if they did mind - you're free to do with it as you like. More people using it may mean more input from other users on how to improve future versions - which benefits everyone.
Other people using it doesn't take away from the people who purchased it. They made an informed purchasing decision when it was a paid-only plugin. They have had a head start in using it. Maybe you'll offer them priority support or access to future beta versions before they're available to people who have not paid?
Maybe give the people who paid a namecheck in the readme file?
Out of interest, what is the plugin - if you don't mind me asking?