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By default it will create a new lead -- see detailed discussion here: https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PapiAAC
The email is a primary key so new email would equal new lead.
There are some ways around this via the API. To solve this using a Marketo form, one good suggestion I saw is to create an alternate field (e.g., "new email address") and have the lead update that.
I believe you'd have to pre-populate a hidden field with the old email address to identify the correct record, then when the form submits with the new email address, have a campaign that runs to update the lead email address for that record to the token for the "new email address" field.