What you should do is log on as the 'owner' of the database.
To figure out who the 'owner' is, go to
Tools->Security->User and Group Permissions,
then go to the "Change Owner" tab,
then select "database" as the Object Type
Then the "Current owner" will be listed.
Now, log into the database as the "current owner" of the database, and try the conversion again. If you don't have the password for the "current owner", well, you should.
If everything keeps failing, you need to strip all security out of the database in order to do a successful conversion. The easiest way is to export all database objects out to a new database file--when they are copied into the new database, their specific permissions are stripped, thus effectively removing security.