If there is one thing you would never want to do is release a game as shareware and then make it freeware.
Sorry, but that's bullsh*t. As the owner of the application, I have the right to change the policy whenever I want, so there. I don't think that the hassle of sending out registrations is worth the money that I get from the payment service, and I did send out quite a few I must say.
I think it's worth to release a $ 19.99 / $ 24.99 with more levels, singleplayer missions, but not a $ 10 game with less than that.