It's important to keep Service Pack version in mind in your backup/restore strategies and operations for system databases like MSDB.

System databases can only be restored to a SQL Server version identical to the one used to create the backup. This condition goes down to the Service Pack level. A system database backup made with SQL Server SP1 cannot be restored on SQL Server SP2. If you are in a situation where you need to restore from a backup created with a different SP, you will need to do a fresh install & restore the backup before upgrading to the latest build.