Marketo de-dupes based on the email address. If you do what you described above and change the email address in your exported file and re-import to Marketo, a new lead will be created and will not update the existing lead's email address. If a lead with the same email address exists upon re-import, Marketo will update that lead. You can update other fields using the email address (as opposed to the ID) but you'll need to edit the email another way.
One of our issues is that we have multiple leads with the same email address, so if we update the using by looking at the email, it will find 2-3 leads with the same email. thats why i wanted to use the Marketo ID to find the correct people.
You'll have to use the API to update by ID. It's not that hard, but you'll need the help of a software developer. You can export a list including the Marketo ID, but you'll need the API to save back the updates.