For a normally healthy person,
AFAIK, the main "killer" difference between an FAA Class 1 and a JAA Class 1 is that the former has the audiogram, where
both your ears have to be within specified limits at each of four frequencies (the two ears are tested separately).
A lot of people fail this test on one ear; probably most old airline pilots would fail it on both ears but they are OK because the renewal limits are a lot easier.
The JAA audiogram appears pointless because somebody who can't hear well enough to operate a plane would instantly fail the FAA hearing test. Also we don't see U.S. 747s plummetting everywhere.
I have "bumped" into several people who reported working on a conversion process but never heard any more.