Hmm, depends. Which is totally not the answer you are looking for, sorry, *grin*. I'm usually okay with small changes, things that I'm not quite sure of anyway. Where did they spend Christmas of '99? Ya got me! Make up whatever you like. Want to write the story with everything exactly the same except that Lance started the band instead of Chris? Convince me. Make it the point of the story and don't just drop into the sotry as a starting point to go off on some tangent and not address how that affects things.

Now everyone's opinion will vary and I am sure that if I knew every little detail of their careers, I might be more sensitive to canon, but as long as you have reason to change something and if has real meaning to the story, I say go for it.