[pertique]

I had the same problem when I switched off Google. I didn't have a ton of data, and I just wanted content for past search purposes, so I didn't dig into how the data would be transformed but I can at least offer my scuffed solution.

I installed a third-party client (Thunderbird, but I imagine any would work) on a local box, signed in with both emails, and just copied the mail over from one to the other. Low-tech, but it worked quite well. I may have forced some local cache/download for the original email, but I can't recall. I'll check later if it preserves headers and the like. I assume it would, but it wasn't that important to me.

I actually thought about writing at some point about the process of getting off gmail and all the funny things I ran across.