There are several threads on this. Please search.
I would personally just create a brand new instance and tie it to the correct SFDC Org. Support can help you do a transfer of data to new Marketo, however, there is some data loss involved.
There may be a Session related to this. Happy to chat if you find me on April 30/May 1.
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I am not coming at summit but my experience is that migrating data and programs from one Marketo instance to another is quite painful, so I would try to limit that and consider option 1 first.
Marketo is now offering some professional services that ease these types of things, so you might want to talk to them, too.