Gigabit

Hi all, I recently upgraded from 400 mbps to 1gbps. I was getting on average 477 mbps with a upload of 25. After upgrading I noticed that my download did not increase at all but upload went to 35. After contacting comcast I was told to swap put my modem. Still no increase in download speed. Than a tech came out and ran tests. The tech said he will have someone come out and look at the line. The line is the same coax that was installed 10 years ago. I've tried numerous ethernet devices that are capable of 1gbps and I used multiple wifi devices that I have seen receive around 800mbps on att fiber and spectrum fiber. So I know what my devices are capable of. How can I speed up this diagnosis process. I beleive comcast needs to change out the line. Any help is appreciated thanks.