Your problem is simpler in one essential way: you are neither adding nor subtracting any notes, just repitching existing notes. I feel fairly confident I could solve yours -- use map-some-music to find every NoteEvent, and for each one, read the old pitch, look up the corresponding new pitch, and write the new pitch.
The writers of \addNote and \pitchChange found ways to deal with having the replacement have more notes than the original did. "Convert a note into a chord with all the same slurs, dots, etc etc hanging off it" still eludes me. It seems that articulations get stored in two different places according to whether they apply to a note or to a chord.
GRB
Maybe check this thread out. It mostly worked for me but I think your problem if solved will help me even more!