a) If you move your melody by one semitone without locking to scale, your melody remains exactly the same; it just plays one ST higher or lower. This effectively changes the key (the base pitch) of your melody and you should adjust the rest of your pitched instruments' MIDI accordingly for them to fit together.

b) If you move your melody by one semitone while locked to scale, it'll very likely change the quality of it in many places, as minor thirds may become major thirds, fourths turn into into fifths, stepwise movement turns into repeating pitches and so on.

You're not going to get a progressive metal melody out of a happy melody by locking from Major to Phrygian and moving up or down a ST. You'll have to write a dedicated melody for your key/scale/mode.