I see that you mention that you are using the demo so I assume that you are still in testing mode rather than a published app? If so, you can just select no initial data when you test the app and it will create an admin regular user for you again. Of course, you will have lost all of your other users.
Derby is easy to use but not so easy to manage. It requires a special driver to be able to view the contents of the database directly in order to be able to recreate the LoginNames in the database directly. It is too bad that you didn't believe the error message: Data for attribute LoginName will be lost Are you sure you want to proceed? When it was presented.
Other database formats such as MySQL are far more friendly towards fixing this kind of thing. If you don't have too much data yet, I would suggest using the above solution if you are just using test mode, otherwise the easiest thing is probably to export your bsv, delete your business space, recreate it and import your bsv to it. This will give you a blank database with a single admin user. If you have a bunch of critical date which is probably unlikely since you are only in demo mode anyway, you can google Derby and IBM offers a driver that will allow you to access the data directly. You can search this forum for several threads on this topic.
Cheers,
Pete