I am not sure, if workflow will retain connection to Contact in this case.
What happens if you create lead today and qualify contact after a month ?
Approach 1:
Instead do this :
When you qualify the lead, qualify it to a contact. You will not need a workflow in that case.
Approach 2:
Create a custom contact lookup on Lead entity. When you create contact through workflow,set contact in this lookup field.
Later you can update contact through workflow