It will only delete users that are not fully registered with DNN. All users that are fully registered and working will be left alone.

You can run the first select query to see what users will be deleted before running the second TSQL script.

If you are using the Membership tables to for other applications besides DNN then you don't want to run this.

I guess it would be easy enough to turn this around and add the members from the aspnet_ tables to the DNN tables.