And then she hits on something else I've been dealing with lately, too: changing what you've got. Setting up your keybinds is one thing, but actually changing them can be tougher.
For instance, my Hunter has always had Hunter's Mark attached to the 5 key -- that was the easiest and quickest place for me to put it when I was setting things up, and so I've trained myself, whenever I am about to start shooting a target, to smash the 5 key and move on. But I have Arcane Shot sitting on the 6, and Explosive Shot (when I'm in that spec) sitting on the 4. That can cause problems, as you might have guessed: reach up to mark a target before the fight, accidentally hit the Arcane Shot, and then I get labeled as a noob for pulling early (probably a deserved title for many reasons, but still). So I decided to switch the mark away from there, and put it somewhere else more out of the way.
But instinctively, as I've been hitting 5 for Hunter's Mark for so long, I would reach up and hit 5 again whenever I entered combat -- I had a lot of trouble retraining my reflexes to hit something else. I only really tried to change for about an hour, but at that point I had to run off to a raid, and rather than making mistakes during a raid setting, I just went ahead and moved the keys back.
I presume that if I gave it more time, I would eventually learn the difference (we humans are nothing if not adaptable), but I was definitely surprised to see just how ingrained my reflexes were with the key presses. xella says she wants to change as well, and most people recommend what I would do (and what I'll probably have to do if I really want to change) -- just take the time to practice and re-learn the keys. It's interesting that we can change keybindings in seconds, but especially if we've built up experience over time, it's much harder to change the way we move.