Change key signature *affecting* the notes

I'm attempting (for mostly recreational purposes) to change a song written in major to one written in minor, but in MuseScore when you apply a key signature to music it automatically adds accidentals/naturals to attain the "original melody" even with a different key signature. This makes sense for serious composing, but is there a way to suppress this feature so that applying a different key signature actually affects the way the music sounds? That is to say, the actual notes would be written the exact same, but because the key signature was changed they now sound different. If the note had a flat before and the key signature would add another flat to that note, it would still be written with a flat making it de facto a double-flat.