can the cylinder switch on a 2004 honda odssesy be rebuilt instead of replacing the whole key switch?

If there is an electrical problem the part with the electrical contacts is sold separately. If the key is worn so the switch does not work I'd have the dealer parts department cut a new key using the VIN and their key cutter that uses code instead of just tracing the worn out key. If the key is chipped just have them make a regular key first to try to make sure it works.

Yes. An auto locksmith can replace just the cylinder or I think it's also called the tumbler.