As we've seen with Revan, you can switch from darkness to light, but that decision would not come lightly and you'd need a tremendously significant event to trigger it and the chances for that are not favorable.
I think that if someone who follows the light side decides that they can resist the dark side to the point that they master its use, and NOT be affected by it...they will ALWAYS fall into darkness. That is what happened to Revan, Malek, and Exar Kun even before that, as well as Freedom Nad. They all sought-out forbidden knowledge thinking that they could master it without it controlling THEM...and in the end...well, we all know the answer...
A light-side character cannot go dark without being consumed by darkness, so only someone who has fallen into darkness and turned to the light can master the powers of both light AND darkness (i.e. there is no other "middle-ground" other than being completely neutral like the Voss, there is no "gray-line").
I think that the true idea behind "grey jedi" is that they are warriors that have devoted themselves to maintaining balance between GOOD and EVIL, NOT meerly balance between light and dark. So, technically speaking, the Grey Jedi aren't somewhere BETWEEN light and dark, they're Warriors that are sworn to act as deter-rents against evil and to stand-up for what they believe is right.