I'm guessing that what's happening is that your BB was attached to one mail server and now it's on another?
It's possible that not all of the old message is on the Blackberry, as messages are downloaded one piece at a time, and attachments are rendered by the server.
You could try forwarding the message to yourself from the BB. The initial send will probably fail with a red X, but select resend and see how much of the message you can forward to yourself. I don't hold out much hope though.