In Sitecore 8.x, when a contact is identified, it's done so just on a single string like an email address or username.
In Sitecore 9.x (more specifically xConnect) when a contact is identified, there is an Identifier Source and Identifier. The source being some arbitrary string value to signify the source of the identifier.
When upgrading between Sitecore 8.x to 9.x, what happens to the contact identifier and what is the "source" of the identifier after a xDB Migration?