Yes it is possible to do a per-domain conversion. Keep in mind, once you upgrade a domain it cannot be moved back.
1. [17 Server] Add a domain that's the same name as the one you plan on adding.
2. [17 Server] Shutdown the server.
3. [15 -> 17] Copy the domain folder with all the data ontop of the domain you just created on the new system.
(OLD server)C:\SmarterMail\Domains\<domain> -> (NEW server)C:\SmarterMail\Domains\<domain>
4. [17 Server] Boot up the server, log into a user in the new domain it should be working.
5. Make sure to fix your DNS and addressing to go to your new server.
Let me know if you have an issue with any of these steps. I would log into a user in your 17 box and verify it's working before getting rid of the domain on 15.
Matt Petty
Senior Software Developer
SmarterTools Inc.
www.smartertools.com