So there isn't much to "migrate" over from SMTP5 to the Brightmail Gateway (BG). I think really the only thing you could pull over would be your allowed and blocked senders list. The format of this hasn't changed from my understanding so you could do that if you want to.
Your compliance filters and such are going to act differently than SMTP 5, so you may need to reevaluate these anyways. With the multi verdict engine in BG versus the single verdict engine in 5 is going to make many of your compliance policies act differently.
My best suggestion would be to get BG in place and get familiar with it if at all possible. Then move over what you can and go from there. Our only guides are the
implementation and install guides.
Let us know if you have any questions along the way as we would be glad to help!
Thanks!
Here is all the documentation for BG.
http://www.symantec.com/business/support/documentation.jsp?language=english&view=manuals&pid=53991