Change GUID Contact Key to SalesforceID for synchronized DE records without deleting the record in Salesforce

Extract all the records that have UUIDs as contact key by following the Permanently remove Contacts or Subscribers and associated data from Contact Builder Knowledge Article.

After they're extracted, pause the synchronize data sources and initiate a Contact Delete on these records. Wait for the Contact Delete process to complete. Once the delete is completed, resume the sync. This will introduce the records as SalesforceID associated with the contact key. Note that if you resume the sync before the Contact delete process is completed, it reintroduces the UUID's as contact key for the records that are in a Suppressed state. You can track the deletion process by following the Delete Contact Information from Your Account in Contact Builder documentation. 

If the contacts were successfully deleted but still have a GUID as the contact key value in the Synchronized Data Extension, perform a full refresh by clicking the refresh button on the synchronized object.