not really true ... you will find flyers/postes all over subway a week before and alternate ways to go about it... never had routing issues...
NY subway is really very methodical and once you get the logic, you are on a roll, trust me!:)
No she's right, while they do put signs up beforeand, sometmes somthing will happen on extremely short notice..i've exerienced it many times coming home on a weekday, if im sitting on the express, it iwll go local or vice versa.. but as far as lines getting switched around or rerouted that rarely ever happens on short notice