"So, would it be accurate to state that, in general, one can substitute a chord for any other as long as the two are vertically aligned?"
Absolutely.
When a single voicing can be interpreted as so many different PMG's (and we haven't even touched on AMG, CMG, etc - a whole different ball of wax lol) then they all become synonyms of each other.
The really cool (and I think most insightful part) comes from mapping out a bunch of these kinds of changes using the "close to distant" chart and looking at how various progressions relate to each other as far as Lydian Tonics. I think Motherlode put it best with his analogy of the Lydian Tonics in a progression "hanging like bulbs on an Xmas tree" (or something like that).
Examples:
The C Lyd voicing I mentioned last time (F#-C-E-B)
Lets move around the "close to distant" chart and build some other voicings:
G Lyd voicing (moving in the direction of 1#): G-C#-F#-B
D Lyd voicing (moving another #): G#-D-F#-C#
Now, how do we:
#1, label each voicing? (What PMG do we want to associate each chord with?)
#2, arrange the voicings into an actual progression?
Also, what if we want to use a voicing to function as MORE than one PMG of the current Lydian tonic?
Here's an easy but cool interpretation:
G Lyd voicing: G-C#-F#-B - use this as an A13 CHORD (PMG II)
C Lyd voicing: F#-C-E-B - use this as a D13 CHORD (PMG II)
D Lyd voicing: G#-D-F#-C# - use this as an E13 CHORD (PMG II)
Voila! Chord changes for a typical "Blues in A" but with each chord "vertically" connected to its Lydian Tonic!
NOW, think about this: What other PMG sounds can you get for the C Lyd voicing, the G Lyd voicing, and the D Lyd voicing?
What if we went in the OTHER direction on the "close to distant" chart and threw in this:
F Lyd voicing: F B E A (moving in the direction of 1 FLAT away from our first voicing that came from C Lyd)
Let's take those 4 voicings and start with this:
F Lyd voicing: F B E A - use this as a G13 CHORD (PMG II)
D Lyd voicing: G#-D-F#-C# - use this as an E13 CHORD (PMG II)
G Lyd voicing: G-C#-F#-B - use this as an A13 CHORD (PMG II)
C Lyd voicing: F#-C-E-B - use this as a D13 CHORD (PMG II)
Put it togther as | G13 | E13 | A13 | D13 | and you get a 'typical' turnaround that many people would label as a "I VI II V" using functional harmony, but it's REALLY just a series of II PMG CHORDS from various Lydian Tonics.
Now have fun with it:
F Lyd voicing: F B E A - use this as a G13 CHORD (PMG II)
D Lyd voicing: G#-D-F#-C# - use this as an E13 CHORD (PMG II) AND as a Bb7#5#9 CHORD (PMG +V)
G Lyd voicing: G-C#-F#-B - use this as an A13 CHORD (PMG II) AND as a Eb7#5#9 CHORD (PMG +V)
C Lyd voicing: F#-C-E-B - use this as a D13 CHORD (PMG II) AND as an Ab7#5#9 CHORD (PMG +V)
This gives you the "Tritone Sub" idea but with much more organization:
| G13 | E13 Bb7 alt.| A13 Eb7 alt. |D13 Ab7 alt. |
The Lydian Tonics DIDN'T CHANGE AT ALL. The voicings of the chords DIDN'T CHANGE AT ALL.
What would happen if these were Lyd Aug voicings? Or Lyd Dim voicings?
Think of how much further you can take this, especially through an entire tune