Hi there, 

Welcome back to the Community. I'll be glad to share more info on how to add an existing estimate to a project without starting a new project. 

To do this, open the estimate > change the Customer name to the Project name > click Save and close. Doing this won't delete any of the line items. For more info on Projects, check out this helpful article:Set up and use the Projects feature.

Let me know if you have other questions. I'm here to help. :)