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SSL certificates are closely tied to the host names involved, that includes both INTERNAL and EXTERNAL.
Therefore you need to look at what you are doing very carefully as part of the overal migration. You cannot treat them as just a single step that needs to be done to move on to the next one, because if you do, you will probably break a lot of functionality.
What is your migration plan for the data? Staged or big bang? Do you need to keep access to OWA, Activesync and Outlook Anywhere throughout?
As such, both answers, using a new certificate and exporting the existing one are correct, but only in the right circumstances.
Simon.