Upgrade SQL Server 2008 Express to SQL Server Workgroup 2008 Volume License
You can use the copy database wizard to move databases from one database instance to another.  

Right-click on an existing database, choose TASKS and then at very bottom, is 'Copy Database'. 

I would recommend the detach / re-attach database option, I don't like the other method, it's not reliable enough.

You'll probably want to test a couple of database detach / attach operations.

Detach takes the database offline, and makes it portable so you can change the volume, before re-attaching the database in a new location.