Automatic box

[Warrick]

Hi
My '74 Audi 100 Coupe S that I am building for historic racing here in South africa is almost done.

Last weekend I tried to reverse it out of my steepish but not too bad driveway. It went halfway, jumped out of gear and now I cant get it to go into reverse. Drive and 1 + 2 are still fine?

Any ideas? Am I going to have to pull the box out? HOPE NOT!!!

Also, how simple would it be to change it for a manual g/box? I have a complete donor car with engine and manual box.

Any advice appreciated.