According to this Knowledge Article
When you then convert the Lead to a Contact, the values change and the ContactID replaces the LeadID and this then update All Contacts (view in Contact Builder) too, to ensure the record transitions smoothly.
The questions are: Does ContactID replace the LeadID in Synchronized DE Lead_Salesforce? Which field is being updated in All Contacts (ContactKey?) If so, is SubscriberKey from All Subscriber list being updated?
Please provide more details about the behavior of converted Lead in synchronized DEs, All Contacts, and All Subscribers lists. Thanks!