[phone man]

Is your intention to keep Comcast and add over the air antenna service to the living room and master bedroom or are you replacing Comcast with the antenna?

Is this coax stapled down? From the attic, can you physically trace the coax to where it enters a wall cavity and have someone verify it's location while you rattle it?

If all other coax cables are carrying Comcast and this is the only one that is idle, you should not lose cable service in any room when you cut this coax, correct?

There is equipment that could verify that this is the coax you're after before you cut it. A telco or CATV tech would have access to such equipment if you know anyone in the business.