The converted Lead is Deleted in Acoustic Campaign and replaced by a new Contact. The new Contact generates a new recipient ID, and then the new Contact is pushed from Acoustic Campaign to CRM.
After the new Contact is pushed to CRM, a merge job is initiated in Acoustic Campaign that merges the marketing history from the recently deleted Lead into the new Contact.
Essentially there is a winner and a loser, the loser is deleted, which in turn deletes the synchronized record in Acoustic Campaign. The winner is a modified SFDC record so any new mapped field values are synchronized with the remaining Acoustic Campaign record that is associated with the merge winner. Additionally, the integration includes any marketing history from the loser of the merge onto the winner via the Acoustic Campaign activity insight URL.