Is it possible to modify AI planes waypoints,altitudes etc. during flight? With IPC or ABL or any method?

Download ttools202.zip by Lee Swordy and read the fine documentation provided on what can/can't be accomplished with AI. You can't modify the waypoints but you can assign the flight crusing altitude.AI aircraft only fly within approximately 90nm of your aircraft. They follow the flight paths determined by routes, waypoints provided by the other parts of the flight simulation.W. Sieffert

Yes it is possible, in the sense that an interface exists. But no, this interface is not readily available to developpers...