Hi Damian.
Well, the subscriptions you have over at PayPal, will remain there and collecting the coming payments until cancelled, regardless of the plugin you have on the site.
What changes is what happens when the subscription ends.
The subscriptions will either have a notification URL set individually, which if set are probably EDD’s. Or PayPal will be using its default notification URL, which you can update from your settings, of course.
s2Member expects certain variables (i.e. custom value), and being notified to its own notifications URL. This you can probably sort out with an IPN forwarding script that picks up the notification to the old URL, prepares it for s2, and forwards it there. There’s a starting script we provide, that you can customize for your specific situation. https://s2member.com/?s2member_file_download=ccaps%2Faccess-s2member-ccap-pro%2Fs2m-pro-extras.zip
With s2Member Pro’s user importer, you can update your user profiles in bulk to have their PayPal subscription ID in their profile, under s2’s field for it.
Those notifications are important if you want to have s2 remove access automatically when the subscriptions end. If you’re fine with demoting those users manually, then you don’t need to worry about integrating the previous customers with s2.
New customers subscribed thorough s2, will notify s2 when payments and subscription end happen.
Does that answer your question?