Is there a way to combine marketing user email events from sendgrid to an existing user? 

We send marketing emails to people who showed interest in our product. Sendgrid events use email as user id in amplitude. When they convert, i want to tie those events to user that’s created. We are NOT using email as amplitude user id, but a different unique id. After they convert, is there anyway i can associate those marketing marketing email events to now converted user on amplitude? Since user id’s are different in amplitude, this seems impossible…..

Thank you in advance!