www.Photosig.com is a good example of a service that started out free and went premium. Now they have both types of membership, but obviously prefer people to pay, and give "extras" to those who do. I think photo.net works on the same model, and pbase.com either does, or did ... I haven't used them in years.
People are never happy about starting to pay for something that they've been getting for free, so tread lightly. Don't ask anyone to upgrade, instead, simply tell them about the new features that, due to server maintenance, you're unfortunately forced to charge for. Or something like that.
I publish a lot of free software, and will build "enterprise versions" of them. If you haven't worked it out by the time I make this happen, I'll let you know how it goes.
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