This sort of thing always annoys me. I could just as well say that Brian's Gothic Symphony changed music, because before he wrote is, there was no Gothic Symphony by Brian in existence, but after he had written it, there was. Furthermore, to use that awful phrase, it changed music FOREVER! Because having been written, it can't be unwritten.
More to the point, though, any argument about whether or to what extent Brian's first symphony or the handful that immediately followed might be said to have "changed" anything at all is surely undermined by the fact that none of them did so at all because they weren't performed until many years later so no one could have heard them except Brian himself in his own head.
There needs to be a realistic distinction drawn between the perceived and actual effects of certain music just as there does between the greatness of a particular composer and the extent to which his/her work might be considered influential and for how long and where,
pace the Boulez thread in which one could argue that Dutilleux is a greater composer but a less influential one (which is not meant as any kind of pejorative judgement).