How are the existing ratings converted into the new flags?
They're not. I decided that too many negative ratings aren't flag-worthy, and there's no way to automatically determine it. If you believe that a past negative rating is flag-worthy, you'll need to create a flag.
Can everyone create a flag? I have seen add flag option in users profile. Does this have any affect by DT member? Or everyone can create flag and if get support, it will be active.
Got it.
Only users in your trust network count as supporting or opposing flags. For guests, the default trust network is used.