Imo, the bluesy feel this changes transmit are caused by the I7 IV7
passages:
| G13 C9 | G13 C9 | G13 Am7 | G/B G13sus G13b9#11 |
| I7 IV7 | I7 IV7 | I7 IIm7 | I I7 I7 |
| C9 Bb13 | Am11 D+7 | G9 C9sus | G9 C9sus |
| IV7 subV7/ii| IIm7 V7 | I7 IV7 | I7 IV7 |
So this is kind of a variation on the blues progression.
The original changes were:
| G D+9 | G D+9 | G Em | Bmi Emi |
| I V | I V | I VIm | IIIm VIm |
| C9 | D+ | G D+7| G D+7 |
| IV7 | V | I V7 | I V7 |
So imo, the re-harmonization took on a totaly diferent aproach on the tune
(which was pretty simple harmonicaly), and introduced a bluesy feel (which
was already sugested by the original IV7 on bar5).
So imo, these aren't substitutions, because the overall harmonic structure
is completely changed. A substitution would preserve the general sound of
the chords. Here we go from a I-V type of harmony, to a I7 - IV7 which is a
whole new aproach, and a true change in the feeling inspired by the harmony.
You can't normaly use a I7 instead of a Imaj7, but if you do, you'll notice
the bluesy feel this conveys. And most of the times, this changes the nature
of the tune - so you must want this effect! Same with the IV7. Its preety
hard to figure out I7 IV7 passages in styles other than blues.
In a nut shell, this may be an interesting reharmonization per se, but I
don't think there are substitutions here you can aply to other tunes.
Please correct me if I'm wrong.
Pedro