Raggy (and Joe) you can use this principle for most songs to simplify the chords. Normally it's done the other way, replacing the simpler chords with the extended ones, but you can reverse it:
Replace With
maj7, maj9, 6, add9, 6/9 major eg Cmaj7 -> C, C6/9 -> C
9, 9b5, 9#5 dominant 7th G9b5 -> G7
13, 13b5, etc dominant 7th G13 -> G7
min6 minor Dm6 -> Dm
(I'd normally try and keep min7 as min7, but if you can't, replace it with the simple minor chord.) The m7b5 (eg Dm7b5 above) has the same notes as a minor 6th chord a minor 3rd above (Fm6 for Dm7b5) so you can replace it with that or just the minor a minor 3rd above (Fm for Dm7b5); or you can try the simple minor (Dm in this case)
The dimished chord is more of a problem since it can have several functions. Often it's a passing chord in a sequence like: C C#dim Dm7 G7, where you get the nice chromatic bass run C-C#-D, in this case you can just keep the original C chord going: C C Dm7 G7. However the dim7 chord sometimes replaces 9b5 chords - see other Mudcat discussions on the diminished chord for this.
The augmented chord above is written as E+, but this is the same as C+ (the version I have in Eb gives the sequence as Eb Eb+) and you can keep the C chord going if you don't want to use the augmented.
There are other chords (eg various types of 11 and other minors) but the ones above are the ones you'll come across most often. If you use the simpler chords the worst that's likely to happen is that the extension or altered note is the melody note and you'll lose it from the accompaniment chord.
If you can do the extended and altered chords it can make the accompaniment sound really good, but be aware that often you need to find the right place to play the chord to make the sequence sound good - often to get good bass movement. If you're not familiar with the sequences you may need to try various fingerings of the chords to find a satisfying one.
Mick