Sounds like you're not upgrading the MSDE, but moving the DB over? I would say, create a new database on the new SQL with server config util, or createdb (with the same name as your current DB). Then, restore your old BES DB to the new SQL, writing over the new one.
Reason...when a BES DB is created, there are some master db entries created, etc. You need to get that stuff in the new sql. Then, just cut over the BES boxes.
look at this article too: Livelink - Redirection