In general, theres nothing wrong with doing a water change, especially if you thik your fish is ill. I dont think it will affect your cycling in a bad way, since the fish is/are producing waste all the time. And you want to be sure to keep your levels from going all wacky. Id do a water change now (25-30%) unless youre planning on going to the petstore in an hour or so. Use a gravel vacuum (siphon) if you have one, if not buy one soon.
FYI: Either Petco or Petsmart sells the API master test kit and matches their internet price which is $15 bucks or so LOWER than in-store....someone on here must know which it is!