You can replace one or two fletching at a time.
You
can do that, but it's best to refletch the arrow with all new vanes. You don't have to refletch all your arrows if you do one, unless you go back with vanes that are radically different than the vanes on the other arrows. In that case, then you'd better refletch all of them or you could wind up with arrows that won't group together.
Honestly, I've never been able to get an old vane off an arrow without completely destroying it - assuming the vane was properly glued down in the first place - so I don't think your idea will work. Vanes are cheap, as long as you stay away from the fancy stuff.