Honestly, I don't see a big problem in changing fursonas. But then again, this is coming from an artist who has at the very least two characters she alternates between. Changing a character is not something bad. People change as they get older, and I'm sure their fursona changes accordingly. People have to grow. If the character was significant, however, it doesn't matter if you have changed fursonas, the art or commissions you got should still be meaningful because you can reflect on what was going on in your life or what was important to you through said character, even if they are old. But that's just my opinion or how I view it. Not everyone does this, but there is such thing as a too much.
Too much, to me, is having a different fursona every week. Or just blowing through ideas. I don't have much room to speak because I used to create characters like mad, but I always returned to Rigor / Nyx in time. Some furs have to do this to get a handle on who they are, or who their character is (I did this when I was a newer fur), but it gets old after a while. As far as other people changing, hell, it might sound cold, but the only way it would bug me is if they were changing while I was in the middle of doing a commission for them, I had already done the majority of the work, and they wanted me to make changes accordingly or draw their other "newer" character. Otherwise, they can do what they want. I got the money for my work, they can do what they want with the art so long as they don't claim they drew it, ect.
...BUT. That's just my opinion.