Arris Surfboard SBG8300 and Xfinity 600MBPS plan

I purchased this Modem/Router Combo to run the Xfinity 800MBPS internet plan which it has done so flawlessly for a year. I decided that I did not need that much bandwidth and being a senior attempted to downgrade to the Xfinity 600MBPS plan to save a bit, but it said my modem was not compatible due to "Filters".  

My question is, why will it NOT work on the 600MBPS plan but has run flawlessly on the 800MBPS plan. I could understand if I was trying to move up to the Gig plan which I know it doesn't support. But it should run on anything lower than the Surfboards SBG8300 rated 940-950MBPS.