Agreed but I think it is also a bit unfair to penalise a bowler who upon sussing the batsman's intentions intelligently slips the ball 'marginally' down leg side in order to thwart him unless you bowl so wide that it misses the wicket-keeper and everyone and flies off to the boundary. Some credit must also be given to the bowler for not letting the batsman reverse-sweep or switch-hit successfully.
So once a batsman has changed his stance and/or grip, the 'normal' leg-before and wide (within reason) rule should not apply any longer. You just can't have it both ways.
Which leg side, the old one or the new one? If the ball would be outside leg according to the pre-switch stance, then I say let that be a wide. However, the bowler is free to send the ball outside off upon sensing the impending switch. The ball would turn out to be outside leg after the switch, hence unplayable, but still legal because it's outside off according to the pre-switch stance.
Otherwise we could throw all the benefits in bowler's favor, letting the batsman assume all the risk. If the ball could've been deemed legal in either the pre-switch or post-switch stance, then it is legal. If the batsman could be lbw according to either stance, then he's out. The batsman is in effect, making both sides his off-side for the determination of wides and lbw, and hence loses the security of outside-leg rules. The will really kill the switch hit though, since not many batsmen will be willing to bear the greater risks.