Just to make sure I understand you correctly, you were on the legacy product BES express (now Professional Express), and have purchased the upgrade to BES 4.1, allowing you to expand beyond 15 CAL's, and you want to use/migrate your existing DB to a dedicated SQL server instance on another box?
If I'm understanding you correctly, you simply need to run the BB server config panel to change the DB after migration, and you should have been prompted for the SRP when you ran a fresh install. The SRP denotes the maximum allowable CAL's (for instance, if you used your old SRP and tried to add a 16th user through BlackBerry manager, you would get an error stating you'd reached the maximum allowable users.
To really answer you question about the DB's and Policy migration, I need to get a better grasp on your environment soup to nuts...
1.) What version of BES did you start out with? I know you said express, but I'm trying to figure out where you're coming from exactly, as in what platform.
2.) The new server, I expect it's the latest SP - 4.1.4?
3.) What version of SQL server are you running on the new box?
4.) Are all of your BB services collocated on the same machine?
With policy, yes, you can port it over, though it's a bit of a process, and you may find that you'll want to build new policy from scratch, as there have been quite a few additions and advancements on policy in the latest SP's... check out this manual:
When we upgraded I wrote all new policy (using the default as the starter of course).
If you do it right your users shouldn't be impacted.
Hope this helps let me/us know how other questions.