//Is new_note a dotted half note now?

new_note.duration.numerator: half_note.duration.numerator * 2 + quarter_note.duration.numerator;
new_note.duration.denominator: quarter_note.duration.denominator;

//Is half_note still a half note?

half_note.duration.numerator: half_note.duration.numerator * 2
half_note.duration.denominator: half_note.duration.denominator * 2