The question I'd ask myself if I were you is, "What's the COST of allowing all existing users to continue to use the app for free, and charge new users as they

The question I'd ask myself if I were you is, "What's the COST of allowing all existing users to continue to use the app for free, and charge new users as they signup?" If the answer is that there's little or no incremental cost (beyond, say, hosting costs and occaisional support), then I would allow all existing users to continue using the app for free, indefinitely. Your beta users have likely put up with a lot of crap while you and your predecesor ironed out the kinks, added necessary features, etc. It's better to continue in their good graces and have them be enthusiastic evangelists for your product than risk angering them by jacking up the price.