Old recurring payments won't be triggered within XF. They don't contain the data that we expect/can verify to process them. In that regard, they'll expire as if the payment wasn't received.
However, the payments will still happen in PayPal. You'd either need to commit to manual extension to keep these recurring subscriptions working or you'd need to stop the recurring payments within PayPal and have people resubscribe.
As a side note, depending on how you did the user upgrade, it may not be entirely convertable to XF. We can only properly convert secondary group changes as that's all we have a concept of. If the primary group was changed, we'll import the log of the change, but the group change can't be automated on expiration.