I think they should re-create everything to suit the film, especially sound effects. Sounds play very important part in making the world come alive and honestly I don't wish modern take on Knight Rider sounded like a rehash of 1980s sound effect libraries.
There is one major exception, though; The scanner sound has to stay. At this point its part of the essence of Knight Rider, in the same way Doctor Who's TARDIS has to sound like that, or the way Godzilla roars. Like with those two, the sfx team can tweak and/or modernize the scanner sound, but the base has to remain the same. For example they could add a little bit of 'Matrix code rain' style effect to the background, or add greater change of volume in the pulse, stuff like that, but it still has to sound a KITT scanner.
As for the theme song, well, I loved the way the new show handled it and the new movie should follow the lead. I know, the new theme didn't sound much like KR, but it represented the show really well; It had the same base rhythm and pulse driving the song, yet the sound and melody were different and more modern, just like the show itself. What ever they decide on the theme song, they should avoid the easy route of just jusing a techno/industrial/rap remix.