Ok, so you mean routinely replacing people - again, it happens, particularly with acts that are older. It just depends on what you feel as a fan you can stomach. Is Kansas really Kansas without Livgren and Walsh? Not to me. But then again, they ARE Kansas, they do tour as them, and loads of people go see them. i mean, replacing guys in the band who don't do much of the writing, or whose sound isn't critical to the band's style really, at least to me, is what it is. But its when you replace people who are responsible for the uniqueness of what made a band great to begin with, is where the trouble is.
But again, its all simply subjective. I mean, Judas Priest has survived with Rob, then without KK, and now without Glenn. I am not a hardcore Priest fan, but I am a fan. Is it different? I am sure it is, but at my "level" of fandom of that particular band, I doubt I will be able to tell a difference when I see them this month. And that's not a disrespect to Glenn or KK. Its just that I am not as intimately familiar with their catalog and band to really tell the difference. With Queensryche, I certainly am, and have clear lines on what I do, or do not, support, and what I feel about it.
But it all depends on the individual and their experiences.