First we need to backup MS DPM 2012 database.therefore we can use dpmbackup tool which is coming with dpm 2012 itself.
Syntax of the command(if we are using different sql server, we can mentioned the server and instance also).
DPMBACKUP –DB –TargetLocation e:\dpmbackup
Then we can remove dpm 2012 and sql existing installation from the server.( remember to retain the existing disk based backup)
As a third step we can install dpm 2012 retail edition on the same server.
Now we can restore the dpm database
once we finished the backup restoration, we have to sync the db using same command with –sync parameter