There have been many contradictory observations about the slider and whether or not it is, as you say, harmful to the elbow. When I played---many moons ago---I learned to throw it at the age of 16, and in a very short while it became my strikeout pitch. Maybe the fact that I threw sidearm exclusively had something to do with it, but I never had any problems.
You don't say how old you were when you tried to learn that pitch---maybe it was too soon, I don't know, but I believe there's a minimum age at which one should try to work up such pitches. Okay. Now, as far as a changeup is concerned---here's an interesting point to ponder. My pitching coach, an active major league pitcher, told me once that just about any pitch can be turned into a good changeup. It has to do with the grip---you loosen up on it---and with holding the ball further back in your hand. For example, the palm ball, which I acquired at an earlier age than the slider. Now, I didn't have a fast ball to speak of, so I threw the palm ball like a curve. I also picked up a knuckle-curve (and I do believe that Mike Mussina picked his up the same way; like me, he couldn't do anything with a plain knuckleball because of that sharp wrist action you use when throwing a curve). One thing to remember---you have to throw everything with the same arm motion and the same arm speed.
So go ahead, experiment---you're likely to find something that will work for you as a changeup---and don't worry about the slider. Incidentally, my coach, when he showed me how to throw it, said "Throw it like a curve, but roll your wrist, don't snap it." That was all he said---and I got the hang of it fairly quickly, no trouble at all (except for the batters who struck out trying to get a piece of it!). It became my best pitch, the one I could go to when I had to go for the strikeout. Maybe later on... 8)