I have a standard height, elongated commode. Can I replace it with an elongated, comfort height commode without changing the plumbing?

Most likely yes, you may need to purchase a longer water inlet hose.

Agree with Catonsville... should only need to change out the water supply tube. Just be sure to get the proper "rough in" toilet... either 10", 12" or 14". Here, to determine the proper rough in toilet to purchase, measure from the FINISHED WALL (not the baseboard) to the center of the bolts at the base of the toilet bowl. If you measure 9-11", you purchase a 10" toilet, measure 11" to 13" you purchase a 12" toilet, and anything past 13" means you purchase a 14" rough in toilet. Most toilets at home depot, Lowe's or Menards are a standard 12" rough in. 10" rough and 14" rough are available but do cost more, on average.

Good luck!

Mark