Personally I cannot recall many instances where Philip Glass has drastically reworked a past piece. Literally, there's a snippet here or there like 4 bars with a doubling he wanted in the third act of Satyagraha or an editing of the orchestration of a bit of his Sixth Symphony.  Apart from correcting a wrong note, usually he leaves things alone and channels his efforts to new projects. 

This is a common way of working with creative artists. Some really self-critical artists are limited in their ability to release a less than perfect work, constantly feel the need to revise and re-work, and find it ever so hard to cut the proverbial cord.

I once asked Glass about this when the conductor of an opera asked him about a certain tempo, Glass responded that it was his opera now, that he had written it thirty years before, and that he wouldn't always be around to answer questions.  He was more interested in what the interpreter had to say about it rather than telling him how the composer thinks it should be done.  I again questioned how he resisted the urge to jump in and take control.  He responded "It's easy, I've written 90 pieces since that one."  In other words, he harnesses his creative energy on composing and approaches improving on things he did in the past by improving them in the future.

This attitude is still somewhat amazing to me not only because of Glass' ability to fight against that normal inclination to want to control things, but I'm also surprised occasionally by his comments about things which I consider perfect.  He commented after a performance of Akhnaten a few years ago in Atlanta that "he frequently hears things and wish he could have done better."  To which I respond "How dare you sir!."

So a notable exception and departure of this way of thinking is Glass' Seventh Symphony – the Toltec Symphony.  Glass composed this piece in 2004 though portions of the work derive from other preexisting pieces – most notably the third movement which existed in differing orchestration in the track "The Unutterrable" from Powaqqatsi (1988).  Whereas that track was the basis for the third movement, Glass expanded and re-orchestrated the work.  He also added chorus to certain sections and added a four minute coda to the whole work. 

I mention this because after its 2004 premiere, the Toltec Symphony wasn't performed again until Dennis Russell Davies performed the European premiere of the work in Linz Austria on New Year's Day 2009. Symphony No.7 was the only Glass symphony which Davies didn't premiere outright.  While one can't discount the relationship the conductor and composer have had, now dating back more than three decades, Glass wrote this symphony for another great conductor Leonard Slatkin and received good treatment and rehearsal time with the National Symphony Orchestra back in '04.

Glass attended the Linz performance and during rehearsals made what I consider major changes. The first was to eliminate the singing by the chorus at the beginning of the third movement.  The second move was to excise entirely the coda that he had originally added to the re-worked and expanded Powaqqatsi material. 

The new version of the third movement is now a stronger statement: modern, monolithic, and theatrical in a ritualistic way.  It's true that the original version softened that effect.

So what was the coda?  It was a digression from the mammoth tension that is built up in that wall of sound.  It beginns with the first arpeggios in the entire symphony – then in what I consider one of the more interesting things that Glass ever did, it channels the spirit of Richard Strauss.  I suppose this was originally a reference that we are not, after all, in the ancient Meso-American Toltec society, but we are listening to a symphony for classical orchestra.  To compose this music was interesting.  To have removed it is even more interesting.

The revised ending:

 

Toltec Revised Ending

 

The original coda to the Toltec Symphony:

 

Toltec Original Coda