Okay, here are a few useful tidbits which are helpful to know when one wants to learn to improvise chord substituions in familiar songs or ones you're writing. This also assumes you have a working knowledge of all major and minor chords, and all major and minor 7th chords.

This principle and pattern applies to any key but for the sake of this example we'll say we're playing in the key of C to keep it simple. The dominant seventh - in this case G7th - which would be treated as the turnaround/resolving chord back into C, can be replaced with either: a straight C# or C# seventh; or a Bb7th or 9th; and the root chord, the C chord itself can occasionally be replaced with an E minor or minor seventh - all for the equivalent length of time the G7th chord would have been played.

This type of experimentation lends itself well to improvisation. Before you know it, you discover you've created something entirely different than when you began. It might sound very cool or it might sound completely inappropriate for the song but you don't know till you check it out.

I sometimes amuse myself by taking a real trad tune and throw these types of alternate chord changes into them, just to get a reaction from the listener - and more often than not, it's not a favorable one. But it is fun.