Post installation migration of Caption->Description, etc?

I'd like to recursively populate the currently null custom "Description" field with the current "Caption" data on all photo items, and regenerate all the "Caption" data, using the sanitized version of the filename as per what happens when the form based image uploader is used.

For the latter at least an existing G1 function could be used, but other than that, I'm a bit lost as to how to tackle this one with a (hopfully small) PHP script.

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