Lots of fun AND very time consuming, done a lot of this earlier on, with ATSC coming along life got easier. I would not go into A/B switches etc.

Suggest if only one or two TV involved

Opt #1 If tv(s) have composite in video, just use an old VCR and its tuner for NTSC and use HD tv to switch video source

OR

Opt #2 Buy inexpensive ATSC/NTSC tuner box with HDMI output, use HD tv to switch video source.

The rest is just playing around, and keeping the signal rf level usable from VHF to upper UHF is a challenge.