What happens when you add existing users to Membership subscriptions?

I am working with a site that has added the Membership Plugin. Previously they had about 40 members on a subscription done through plain PayPal subscription buttons. Now that we've added the Membership plugin, we've added the members manually to the correct subscriptions at Membership > Edit Members. The plugin is still disabled. What will happen when we turn it on?

We have annual and monthly subscriptions. Will the members who have been manually added to the correct subscription be starting at the beginning of their annual subscription and then be charged 1 year from when we enable the plugin? Or will they be asked to pay immediately? Do we need to cancel their existing PayPal subscriptions form before we were using the plugin and just start over with Membership?

The main concern is not to be asking for money from members who have already paid previously. Can you tell me the expected behavior of the plugin once it's enabled after we've assigned these members to their correct subscriptions? Thank you!