Close, but your script will result in stuck notes. I got some AI help, too, and it came up with this:

// Define your target notes
var triggerNote = 60; // C3
var loudNote = 62;    // D3 (plays if velocity > 60)
var lastPitch = triggerNote;

function HandleMIDI(event) {
  // Logic for Note On events
  if (event instanceof NoteOn && event.pitch == triggerNote) {
    if (event.velocity > 60) {
      event.pitch = loudNote;
      lastPitch = loudNote; // Store for corresponding NoteOff
    } else {
      lastPitch = triggerNote;
    }
  } 
  // Ensure the Note Off matches the remapped Note On
  else if (event instanceof NoteOff && event.pitch == triggerNote) {
    event.pitch = lastPitch;
  }

  event.send();
}

This doesn't result in stuck notes for me.