question about my Comcast cable box?

There is no point in having an HD TV without an HD signal.

You would have to have a really old Comcast box for it to be 480p (SD). Your new TV is at least 1080p or 4K (you didn't say) which means your old box will not deliver a high enough resolution signal to take advantage of your new TV's capabilities. It's like buying a 2016 car and putting a 1980 engine in it.

Bad quality.. how about poor. At 60 inches it will be poor. The Xfinity is pretty good. Go on line and you can get a deal with internet and cable. The phone is a problem, they almost require you to rent the box for $10 a month. That will go up over the years. Some places only have the triple play even if you do not want the phone (FCC were are you !) but that was still cheaper (believe it or not) then what I was paying for years. Hard to get any smaller packages (FCC where are you !) so you get HBO, SHOWTIME and other 10 year old movies. Oh you can get new stuff, just pay to watch ! MAKE SURE the kids don't "order" the wrestling matches for $$ on your bill. I believe there is a password you can set. The new boxes will be able to record, but in the cloud. You get some massive space and ... drum roll.. you can fast forward on recorded shows. So skip those 4 min commercials. Plus it very easy to record a show and go to bed or record right from the menu just that show or series. With a 60" screen you gotta have HD, the price comes with a HD box. Bad thing.. ha.ha the double or triple play prices come with a contract 2 or 3 year. Watch the triple play for $89. The second year is regular price, but they only state that in infinitesimal small print. AND Showtime or HBO is only free that first year. YOU MUST call them before the year is over and cancel them or more $$ $ per month. It's like leasing a car.. 60 different options.

Your existing cable box will work with your TV but, a non-HD TV channel on a HD TV is going to be disappointing.

It won't fill the screen, and it will look fuzzy.

Furthermore, most of the HD receivers also have a DVR built-in so you can record TV shows and watch them later, when you want.

How much more it will cost you depends on what plan you have now, and what offers are available in your area.

My suggestion is to to to your local Comcast store and talk to them about an upgrade. If you are willing to sign a 2 year agreement, a bundle deal with internet and/or phone, could get you some nice savings (at least during the initial period.) Comcast will even come out to your house, install the new equipment, and show you how to use it.

You will be sorely disappointed in the picture quality.

To compare, hook up an antenna to the antenna input on the TV and scan for channels. The local one's should be in HD, especially sports. Note the difference.