That's right, Prelude was originally recorded (and possibly mixed in stereo?) in September of 1972 or thereabouts, and released in stereo in January of 1973. When CTI got into quad 6 months later, they went back and remixed it for quad because it was their best selling album by a country mile. It, along with Deodato 2, were the first quad mixes that CTI did, so it's possible that Prelude was in fact their very first, so perhaps the quad mixing of that album was a bit of a learning experience in itself.
There are a handful of occasions on the album where a channel falls silent for a few minutes - I wouldn't say it's my preferred quad mixing style, but I think it's fun to have these albums that stand out as an alternative approach to quad mixing. Variety is the spice of life, and all that.
The quad mix of Deodato 2 is far more conventionally constantly 4-channel active, as are all the other CTI studio album quads that D-V have done including Airto's Fingers, Grover Washington Jr.'s Soul Box, and George Benson's Body Talk, so Prelude is really the anomaly. I think it's worth mentioning that the vast majority of Prelude is very much 4-channel active - I think it's just that in this age of everyone being super-paranoid about the calibration of our surround systems that if one speaker shuts off for a minute or two, we immediately worry that something's malfunctioning. Once you get to know this mix, it's kind of fun that a speaker goes silent for a bit, you forget it's there, and then all of a sudden you're surprised by some screaming trumpets or a burst of bongo fury or something.