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You will lose a lot instantly, 25%. Then the other 50% take off as soon as the fees start. It will take about a year to get back to 50%.
Price also was a factor. We went from free to $5 bucks a month. The more expensive probably the sharper the drop off.
Thats my experience any way.
I think the main key is the value added content that the premium members are receiving. If they went strictly to a subscription model they would probably lose most of their users but since they have the basic site and the premium site with the extra features people see the benefit of paying $40 per year.