I "grandfathered" my "active" and regular participating members. Thats the only fair way to do it. Even then some members you may have forgot and that haven't been around in awhile come back and raise all kinds of discontent. But they wouldn't have contributed anyway, depending on their responce I have either given them membership or not, good boys in, bad boys out

simple as that.
The biggest thing is that the regulars, the ones that contribute the most by way of participation usually contribute even though you don't require or ask it of them, even come back and contribute multiple times.