Courtesy of George Elwood's marvellous "Fallen Flags" rail image site:
A 1980 ConRail motive power summary. At that time the E44 fleet was divided into 22 E44A and 44 E44, the "A" subclass including the last ten built (4456-4465) and a scatter of earlier ones, including the first unit, 4400. At a guess the last ten were built as E44A, and the others converted in a program that was cancelled after 12 units.
The main difference is in traction motor (752N1 for the E44A, 752E for the E44) and gear ratio (20:63 for the E44, 22:61 for the E44A) and the nominal horsepower rating (E44A was 5000): I don't think the split between E44 and E44A corresponds exactly to the type of rectifier (life should be so simple!). On the other hand, Conrail seems to have felt that E44A units were four tons lighter than E44, which seems at least consistent with the replacement of heavy mercury rectifiers with lighter solid state.....