On YouTube there are a load of videos where songs have had their keys switched from major to minor and vice versa. For example, YMCA which has been edited to a minor key:
So how were these videos made? How do you change the key of a song?
You can easily change the key of a song by transposing it.
Going from major to minor is a bit more difficult, although there are pieces of software that can do it.
You would need to flat every instance of the 3rd Scale degree, to turn it into a minor 3rd.
To get the full transformation, you would need to flat every 6th and 7th scale degree too, if you are converting to the natural minor.
In short, you are converting the scale they used into its parallel minor.
The song you showed goes into a harmonic minor, as Tim rightly pointed out.
This means that you wouldn’t flat the 7th.
As he also pointed out, the harmonies have to change, but (correct me if I’m wrong) this should be handled by the fact that we are changing the scale degrees.
This is how most of these videos are done, as a far as I can tell, and is especially noticeable when listening to, say the mario theme song in a minor key when you have the sheet music and knowledge of what the Bb to D (major 3rd) interval sounds like vs the Bb to Db (minor 3rd) interval sounds like.
Some songs lend themselves easily to this. 'Silent Night' is an example (little late, but hey ho). Those three main chords -I IV and V are simple to move onto i iv and the choice then between v or V - the latter sounding more convincing. With, obviously, any m3 notes sung as M3, etc.
Changing songs that use, in the major, ii, iii and vi (I'm omitting vio) will bring their own hitches. The tune may need to move away slightly, and/or the harmony (chords) found which match the new tune. there's no direct ' it was an x/X chord in the original, so it'll be an X/x chord now'.
While a piece of music in a major key can be changed to a minor key merely by lowering the third, sixth, and sometimes seventh scale degrees, there will often be places in the melody where this doesn't have the desired effect. For example, if a piece is in C major and part of the melody only uses C, D, F, and G, that part of the melody will be completely unaffected by a change from major to minor. If earlier and later parts of the melody have a lot of E's (which would be changed to E flat), having a portion which sounds just like the major version can seem weird. In such cases, it may be necessary to make additional melodic adjustments. For example, assuming a key of C, the Blue Danube Waltz starts out very nicely with
C C E G G... g g... e e, which will instantly sound minor if the E is changed to Eb. A later section, however, goes
G F, G F, G e.... d, which is harmonized over a V7 chord [same in major and minor] and thus wouldn't be recognizable as minor until the fifth note. Changing the F's to F# (raising them) creates tension and--at least to my ear--greatly improves things. The original melody on the downbeats (filtering out the Gs on the upbeats) is
F F e d which includes a major seventh jump; in minor, that becomes a diminished seventh jump.
I don't think there are any hard and fast rules about when to adjust things on the second, fourth, and fifth scale degrees, but doing so can be an essential part of making a major piece "sound" minor.