Piggybacking using existing household coax
 

I currently have Comcast. The cable comes into the house and at a known location under the ceiling downstairs where there is a splitter sending the signal to two tv locations upstairs. After I install an ota antenna on the roof I plan to have its cable snaked to where the incoming comcast cable is located, disconnect it from the splitter and replace it with the ota cable. Is this plan valid?

Yes. As long as you do not connect cable service to your OTA antenna you can reuse the coax to each TV.

Cable networks are "closed" systems. That means none of the signal in their network can be leaked or transmitted into the air. If that happens, licensed radio services will experience interference. Aviation comms and navigation along with police, fire and other emergency services can be disrupted by leaked cable TV signals.

Also keep in mind that since cable signals are much stronger than OTA, cable is more forgiving of sloppy installers.

Very long cable runs and many splitters inline don't affect the cable signal but can kill OTA.

I've had to rewire houses because of that. I had one house that had 4 different 2 way splitters used in various places after a 4 way on the outside of the house.

remember that the splitter in the comcast box is still theirs.. and you can't mix signal so if you have a cable modem it will need a new wire run to it and back out to the cable box.. but thats easier than running half a dozen wires to all your tvs

also like rick said you may have splits inside your home that drop the power and the splitter that you install will rob power from your antenna

a good thing to do is instead of replacing comcast's splitter with another splitter .. that you use a distribution amplifier .. easy to find on Amazon for about $35 and it will boost your signal enough to let you get it to your TVs .. and you will probably need to run another coax to that distribution amp to power it...

also buy a box off ebay to hang next to the comcast box and mark it OWNED BY HOME OWER or something like that and do everything in that box.. they cost $15 and it means comcast won't come out and cut your wires off .. but if they do you can make them repair it.