I remember distinctly when I first came across the term used in conjunction with music:

Deep Purple in Rock had just been released about a week previously (1970). I was in a record shop and I heard some guy going to the counter with DP In Rock in one hand and DP's Concerto for Group and Orchestra in the other and I overheard him asking the owner which one of these albums he thought was more progressive.

I remember this so distinctly because I was startled by how pretentious this question sounded. I was thinking something like: Wow, this guy's not afraid to make a fool of himself.

So the term was somehow around in 1970, but it also had quite a different meaning to what it has today. Apparently not only has the word itself been around in conjunction with music for a long time, but somewhere along the way it's changed its meaning considerably over the decades.

The transition of the meaning ( as used for music) has evolved from the original meaning of the word itself (as can be seen in any dictionary) to its meaning today, where strangely enough its being applied to an art form that is essentially relatively old.

I think that, mainly because of this transition, the question is unanswerable.

From 69' to 75'...Prog bands were particularly interested in the music of Holst. The Planets seemed to influence King Crimson, Yes, Renaissance, and so many , many others I can't recall. The intro of "The Prophet" by Yes and "The Devil's Triangle" from King Crimson are commonly known ..but there were many other Prog bands from that era who were observing Holst and adding reflections of his work to their compositions. Now that this has been established as a fact which is evident and not some half-wit theory, we can realize that this was a "progression". Between 69' to 75' ...this was a strange & personal developmental stage and a progression into something else. Something "new" to reflect a sound and style ...not yet developed and personalizing the music of Gustav Holst to do it. This was an innovative act that changed the way most musicians had been currently writing. The idea to incorporate Holst and give Progressive Rock a new edge ..made it's way into various catagories of Prog..(eventually). In the life of a musician..this is termed or thought to be a "progression". Many of the original ideas to create Progressive Rock involved taking measures beyond which was thought to be (at that time), an act of insanity  and a very misguiding one. This reaction came in herds and prior to Progressive Rock being promoted. You know how your surroundings might be when you create a new style of music that people are unsure of until it is delivered by "Honest Jerry" on a billboard or radio broadcast. "You work for me..or you don't work at all" type of situation. The huge percentage (I would say), of the Classical world were very stand-offish, offended, and hostile toward the music of ELP and Yes. I recall many of them turning their noses up into the air and commenting how insulting it was to hear Classical music combined with the sound of an organ being smashed on stage.