J.T.
If there is a change in the arrangement, on either side, then a new agreement needs to be made. You did agree to pay her $500 so it should stay that way unless you both come to a new agreement. I would not just short change her without talking to her first about it.
If she really needs the money, what if you let her come over for the week to help you out a bit before the baby comes? Maybe she could help put up some meals in the freezer or help clean or watch the kids while you run errands?