But nowadays the roles of performer and composer are usually separate. If you are a composer/performer, as I suppose I am, some interesting little problems come up. For example, sometimes it is hard to prioritize: what comes first, practicing your instrument or finishing writing that piece? Another problem is that, as a composer, you might be critical of other composers. If you happen to be playing music by another composer and see some sort of problem, what do you do? The temptation is to "fix" the problem. I transcribed a piece by Fauré for viola and guitar and ran across one place where the harmony was really unconvincing: a very feeble cadence. So I changed it! Don't tell anyone, ok?
Sure, as a performer you are always making little modifications, at least if you are a guitarist; because few composers know how to write idiomatically for guitar. Some chords are unplayable, or slurs need to be added. These are usually just technical details. Sometimes, playing a Segovia edition, I have been pretty sure that such and such a chord was just tossed in by Segovia, so I remove it. Call it informed speculation.
If a piece is so inexpertly written that I want to re-write the whole thing, I just won't play it, of course. But in a lot of pieces there are little things that I think should be done differently. A chord misspelled, for example, written with an E flat where it should be D#. Or a clumsy accompaniment that could be more effective, that sort of thing. I'm afraid that even when I was less active as a composer and more active as a performer I had this same attitude. I remember chatting with a composer once, a contemporary of mine, and saying that a lot of guitar concertos needed drastic revisions to the guitar part, to the point of re-writing some passages. He was horrified at the idea. I should have mentioned Brahms' consultations with Joachim when he was writing his violin concerto. I know that there are some clever tricks you can use when writing figurations on guitar and I suspect this is true of all instruments. Sometimes I would really like to ask a player what they are, or what ones I could take advantage of.
By this point you are probably starting to think that I am just too cavalier about this. Who am I to be changing music written by the Great Composers? Quite right! There are a number of composers that I would never have the urge to change because they are indeed great. The only changes I have ever made to an edition of a piece by Bach, for example, were to return to what he actually wrote, removing editorial additions of someone else. Same with Beethoven, Haydn, Mozart and Shostakovich. I'm not saying that none of these composers ever made an error. I am saying that in forty-five years in music I have never found one.
Other composers, though... I would only make a change in a piece of music in which I truly felt that I understood the style and form well enough to be sure that I was making an improvement. I don't do it that often, either. But sometimes...