Actions of converted Users before conversion, using visitorId

We want to know which users have been converted/registered to our site. We are going to use the visitorId of the anonymous user. When that person gets an account created (on an external registration site), the visitorid is changed (since its a hashed value of the userId), so we obviously cant use that info.

We want to use the previous visitorId to track what they did.

How are ways we would be able to achieve this?