Folks have gotten good results from a simple mechanical ten dollar A/V switcher used for component video HDTV, but the results are officially unpredictable.
Here is one thing you can try. Connect the Y output of the DVD player to the #1 video or Y input of the switcher. Connect the (common) video or Y output of the switcher to the #1 left or Pb input of the switcher. Connect the left or Pb output of the switcher to the #1 right or Pr input of the switcher. Connect the right or Pr output of the switcher to the Y input of the TV. Leave the Pb and Pr TV inputs and DVD outputs empty.
Play the AVIA (Video Essentials does not go high enough) 200 TVL resolution pattern and also some other patterns including crosshatch, and observe the quality of upright lines. If you don't notice degradation, chances are the switcher will work OK for HDTV.
Officially the entire video signal path needs 7 MHz bandwidth for interlaced video, 14 MHz for 480p progressive scan, 37 MHz for 1080i and 720p HDTV, and 75 MHz for 1080p HDTV. Degradation is not really noticeable for the 1080i and 1080p with 2/3 of the needed bandwidth.
Video hints:
http://members.aol.com/ajaynejr/whyten.htm