You cannot receive VA disability and active duty pay concurrently, so they will stop it (or retroactively take it back) on the day that your active duty orders begin. For me, it was the day that I started officer training.
It is a two-step process to stop your VA disability. First, contact the VA and tell them on the phone that you are returning to active duty and no longer qualify for the disability payments. (They are familiar with the process.) Next, they will give you a phone number to fax a copy of your orders. If you get paid past the date your active duty orders begin, just put the money aside and prepare to pay it back.