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You do the change in the system module, not the forum module. Click on system admin, then click on the system icon. Then choose preferences, then general settings.Find the comments display view mode in your options and choose flat. That will make the comments and forum posts default for flat view. You will also need to edit your own profile to have comments viewed as flat always. Your users will need to do the same thing. Only the new users that sign up after will have the flat view as default.
