only BES 5 SP2 is supported for SQL 2008 R2, make sure you are at this level
also check this article on how to move from SQL Express to SQL server
KB12247-How to move from a Microsoft SQL Server Desktop Engine (MSDE) database to a Microsoft SQL Server database
if you are not comfirtable with above, uninstall BES from your standby server, then install new BES 5.0 SP2 with db on a sql server, and use migrator tool with BES resource kit to move users over to new BES. minimum down time for users since each migration will take only couple of minutes