Sure, it is nice to discover new parts of the countryside, but not necessarily late in the evening when all you want to do is get home. You don't have any idea of where you are going, you just follow the signs in the vain hope that they will get you back on the right track eventually.
A substitution is the technical term for replacing a written chord with another chord of your choice. Books on jazz theory tend to make a big deal out of this, and the rules they suggest you should be able to apply on the fly will make your head spin. According to my rather unsophisticated line of thinking, the reasoning behind those rules boils down to two simple sets of guidelines: 1) you can change the chord but not the key, and 2) if the substitution fits the melody, it is okay.
Here is All Blues (mp3, pdf, tef) by Miles Davis with an alternative chord sequence. I made this arrangement a long time ago at a time when I was not really aware of the harmonic side of jazz, and I was just trying to figure out how to make the tune a bit more interesting harmonically. The substitutions occur mainly in the final part of the tune.
Now try to have a listen to the Holdsworthian All Blues Intro (mp3) rendered with a synthpad from a midi sequence. You will be able to follow the chord sequence only if you are very familiar with the tune. It is an example of what you can do with a simple structure such as the blues.
I have even recorded All Blues (mp3) with an alternative melody along with the alternative chord sequence! Compare this to the version for solo guitar, or the 'ambient' Intro version, and see if you are able to recognize the underlying chord sequence which is common to all three of them.
There is one substitution, usually referred to as the flat-five substitution, which is so common that I will risk trying to explain the theory behind it.
Here is how it works: take a 2-5-1 in major (ii-V7-I), and replace the V7 with another dominant 7 chord that is six semitones away, that is, the one which is directly opposite on the clock (see the Roadworks for an explanation of this). Example: Em7-A7-Dmaj7 becomes Em7-Eb7-Dmaj7 because Eb (7pm) is six semitones away from A (1pm). An interval of six semitones is a b5, hence the name flat-five.
Here is why it works: the 3rd and the b7th of two chords six semitones apart are the same. Example: in A7 the 3rd is Db (5pm) and the b7 is G (11pm), and in Eb7 the 3rd is G (11pm) and the b7 is Db (5pm). Thus, if you play a Db and a G together, you cannot tell whether you are playing the 3rd and b7 of an A7 chord, or the 3rd and b7 of an Eb7 chord. Consequently, the V7 and its flat-five substitution are interchangable to some extent. What this means in practice is that when you see a dominant 7th chord moving down a half-step it is probably acting as a flat-five substitution resolving to the key a semitone below.