Automatic doesn’t drive backwards anymore

Hello, someone can help us: we have an Audi 100 (C4) 2,8 E, MKB: AAH, automatic GKB: CDT which unfortunately no longer drives backwards. (Ltd. transmission workshop “mechanical error”. My question now is: what is the difference between the GKB? Could we replace a CDT with a CJE and what should be changed with it?