Bastoche: No offense intended. Please accept my apology. I really intended to post the link for the benefit of everyone reading the thread because it is easy to start scrolling down the page of diagrams and miss the link to the Dragon article at the top.
As for changing actions, people have made very good points about allowing players to change actions or not. It seems that it really is up to the DM about how flexible he wants to be.
If you allow this flexibility, though, the rule about "if all you do is move, the square you begin in is not considered threatened" may mean that you may have to apply an AOO "retroactively". As long as the players understand that this can happen, everything is fine.
If you plan to only disengage (move away without attacking) while standing next to an enemy, but something changes your mind (while in mid-action) so that you now want to attack, the AOO the enemy gets should be applied as you leave the square you start in. So if he drops you with it, you never even get a chance to complete your movement, and you never get your attack, right?
Personally, I like IceBear's idea. Players should declare intenions when their turn comes up (after all, they should have something in mind that they are planning on doing during their turn).
However, the player should have a way to change strategy in response to seeing the outcome of a readied action. A Wis check seems fine to me.
Still though, I think there should be limits on what you can do when you change your mind.
Like the PHB says, you can wait until the outcome of your first attack before deciding whether to move or continue into a full attack. This makes sense. (The rules allow this, so there is no penalty here.)
However, there is potential for abuse as Hypersmurf said -- someone that begins a change (a full round action) should not be able to stop a charge partway or convert it to a partial charge(changing his mind) to avoid making a Reflex save because he now notices a concealed a pit in front of him.
I would say that if you move then attack or attack then move, then you have a chance after seeing the outcome of the first part to alter the second part.
For instance, if you plan to double move, you can move your speed (as a Move-equivalent action) and then if you see something that makes you want to attack, you can replace the standard action move with a standard attack action. I might ask for a Wis check in this case.
If you plan to hit a badly wounded enemy (who has already used an AOO) and then move to close with another enemy, but suppose you miss. In this case, you could replace the move you intended with drinking a potion (since you now know you won't draw an AOO because the enemy made an AOO previously this round). I might ask for a Wis check in this case too.
In any case, if the event that triggers the change in strategy was expected or even anticipated, I would either waive the Wis check or give bonuses to the roll. If it were completely unexpected, I would give penalties.