Why not? Your comment about RAI notwithstanding, I don't see anything about disallowing switching targets.
If you have a target and being your movement appropriately, then the target vanishes and you pick a second target. If the remaining movement towards that second target is also correct (for your whole movement, not just from this point on), I don't see anything wrong with that.
You don't move first, and then ask 'Oh, by the way, is there an enemy in that direction?' That's dumb, and is -clearly- not what the rules state you do. You move -towards- the enemy, you do not move in a direction and check if there's an enemy there.
Because, you can't move -a single square- without having a target selected and locked in. So, the charge has a target, the ranger. Otherwise, how would he even know that he was the one being attacked? The moment the ranger bamfs out, you check to see if the charge can still be legally carried through, and if not it is negated. That's per the rules of interrupts. Movement completed is not negated, and that's the exception to that.
Interrupts like this -exist to negate tactics.- You cheat your players if you don't allow powers that negate tactics to actually negate those tactics. You make the game -less tactical- and you make the game -less fun- for the types of players who pick tactical powers such as this one.
You -will- get a chorus of 'If i'd known THAT at character creation, I'd have picked a good power!' if you adjudicate otherwise. The power is designed around the ability to negate, so let the thing do it's job. It's just a monster's attack, and frankly, protectionism over a single monster's single attack is not worth sucking the fun out of a power choice.