Talk to some other pilots and find a good AME to work with. See if you can talk with him/her before you have the medical exam and explain your situation to him/her. A good AME will know what the FAA needs to determine that you do not have asthma (even though you were diagnosed). If you wait to tell them at the exam, they may not be able to help you as they have to report the results. But if you talk to them ahead of time, they can let you know if there is an issue and what you need to do to resolve it.