Greg,
I would avoid just overwriting their previous data. The practice is to treat all changed job leads as entirely new leads because it's an entirely new sales cycle. If you overwrite their email and Company info, you may lose valuable customer data and reporting information.
If you want to import those people as new leads, just create a sheet with their details and import it.
You would use the same process to overwrite - pull a list with their old email, then do:
Email Address | Company Name
with the new Company and it will update the field (unless it is blocked)
but you cannot do this for Email Address because that is the matching database key. I suppose you could do this with LeadID/ContactID in SFDC though. There is an import function to do this.
Lead ID | Email Address | Company ID
Again, not recommended as it will mess up your Accounts.