Why can't I upgrade my Internet Speeds?!

As an existing customer I do not understand why there are so many restrictions as to how you can upgrade your service. A new customer can choose an internet download speed of up to 200mbps, 600mbps, or 1000mbps but for me, an existing customer, I am not able to upgrade from 200mbps to 600mbps. Why is this such an issue? And is there a way I can upgrade to a higher speed since the usage and gaming in my household needs it? @ComcastJessie 

Thanks