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But unfortunately this doesn't give the number of objects that it can monitor with respect to qtree, volume lun etc.
If I remember correctly its possible to convert express to standard edition by doing an upgrade and selecting the edition as standard. But since that you are already in 5.1 and there is no other version to which you can upgrade to my suggestion would be the following.
Create a dfm backup of the current express edition by running the following cli dfm backup create.
Install dfm 5.1 as standard edition.
Restore the backup taken in step 1 in the server where dfm is installed as standard edition in step 2.
Mean time I will try to find if there are some cli to convert the existing express dfm to standard with out upgrade.