Taking out the old disposer

I was having difficulty trying to get the sink flange separated from the support ring (with 3 screws on the underside of the sink).

It seems to have a lip which is locking the sink flange in place. Do I need special tools to get this apart? Any thoughts?

(I would have left the old sink flange assembly in place but the new disposer unit will not fit to the existing s/f assembly...)