If you are going to fly N-registered aircrafts you need an FAA medical.
Dont worry about the medical. The FAA does not have any diopters limits and the medical is MUCH MORE relaxed and down to earth than the CAA. If you look at the FAA website you can locate an FAA AME in your country and do it before you go to the states.
I dont think that the JAA will take away the diopter limits in the near future on the other hand if you already hold a CAA first class medical with deviation, I dont think that you need to worry that much even if they change the regulation since it will most likely only be valid for new "applicants".