If i was writing a song, how would i go about changing from the Minor scale to the Harmonic Minor scale and vice versa?

You sharp the 7th scale degree to get the harmonic minor scale. You put it back to natural to go back to natural minor. Also you use a major chord on the 5th chord of the scale instead of a minor chord. http://neoclassicalmetal.org/index.php?showtopic=18
http://neoclassicalmetal.org/index.php?showtopic=20

I'd say you should rely on your ears when it comes to these things, as the only difference between the (natural) minor and the harmonic minor is the seventh note: in the key of A it'd be G in natural and G# in harmonic.