There is a big difference in changing the price for new users and pitching an increase to your current users. If you are going to price test, do it on landing page visitors (if possible). Grandfather in your current customers at current prices if you can (which might even increase retention), or if the economics prohibit that, explain why the increases are necessary. "We think we can get more from you" is probably not a good reason from your customer's point of view.
This is a very good point. I should have clarified that I mean to charge more for new accounts. If you are going to increase prices on existing accounts do it on a case-by-case basis.
