That is very wrong.
Changing Diplomacy to have an effect anything similar to Intimidate would obligate allowing Paladins (and monks, and clerics, and bards) to respec points from Intimidate into Diplomacy. (It would also strengthen the need for class respec)
The two previous occasions where Intimidate was buffed are not obviously not comparable to the proposal of allowing Diplomacy to taunt, because after each of those changes, Intimidate remained the best (and only) skill to pull aggro. If someone had ranks in Intimidate before, there'd be no reason for him to want fewer ranks afterwards- whatever reasons had motivated him to train it could only get stronger.
The problem is especially clear if you picture an existing Pal16 character who has 9 ranks of Intimidate- how would he react to seeing future pure Paladins getting 19 ranks of aggro-pulling Diplomacy for the same cost in skillpoints?