Virtually all music software can transpose (though not all notation programmes do it correctly in "chromatic" circumstances).

Modern software, including MuseScore, can export and import MusicXML, for which in turn excellent tools for conversion to and from ABC exist, robust and essentially lossless. This is the way to do it nowadays.

Users of NWC can and should try NWC2ABC if they want ABC code. If this does not work, or for users of other old software or recipients of naked MIDI files, conversion from the MIDI file format to ABC can be required. If you use mid2abc, you may have to do some tedious editing of the result. MidiZyx2Abc is a brave attempt to make this work easier; I used it to convert all my old stuff to ABC and then to MusicXML. Hard work, still, but worth it, because files in any proprietary format have time bombs attached. Imagine the thread title were literally true, your old software no longer working on your updated OS — disaster!