In addition to the excellent advice Kolob Canyon has offered, here are a couple more tricks.
The first trick is to use chord substitutions. This is particularly useful if your goal is to make small adjustments in the chords but still keep the same overall feel. Here are a few common chord substitutions to get you going for pop/rock songs written in a major key. (Below, in parentheses, I've written in what these chords would be if your song is in the key of
- a major IV chord (
F maj) and a minor ii chord (
D min) can be interchanged
- a major I chord (
C maj) and a minor vi chord (
A min) can be interchanged
- a major V chord (
G maj) and a minor iii chord (
E min) can be exchanged
In many cases, these substitutions can be applied without having to make any changes to the existing melody. So, you could start out your re-write by choosing a few chords to change, and then you could make minor changes to the melody. One way to do this is to choose one or two notes, and vary them. Here are some techniques for how to vary the melody:
- Reverse the direction of the melody line. For example, if the melody line goes up (
E G A), change some notes so that it goes down instead (
E G D).
- Change the chord tone in the melody. For example, let's say the melody sits on an
E, and the underlying chord is a
C maj. You can recognize that the
E is the 3rd tone of a
C maj chord. Then you can change the melody from the 3rd of the chord to the 5th of a chord (a
G). So now you're changing the melody from an
E to a
- Move a melody note up one scale tone or down one scale tone, while not using any avoid tones.
- Change a small interval/jump in the music to a large interval/jump (or vice versa). For example, if the melody is
C D E, you can change it to
G D E. Note: this trick is fairly limited, and there will be many instances where it doesn't sound great. Use your ear as a guide.
Choosing which techniques (and how many) to use in a single song is a matter of your own personal judgment. It depends on how similar you want your re-write to be to the original. But these are some good ways to start with a song and make careful, incremental modifications until you have your own tune.