Can they change my due date?

I’m 31 weeks and at my appointment today my doctor told me that i’m a small girl and shes growing fast and that she is already in the right position (head down) and that he needed me to come in for an ultrasound to get her weight and measurements but my due date was July 24th and now he’s saying she will most likely be coming shortly after July 5th. He’s a great doctor but i’m wondering is that pretty accurate? or theres still no way of knowing? This is my first baby also.. I just wanna know how prepared I need to be and let my work know that if he’s saying she’s coming early that maybe I should be taking off earlier than planned? I mean I still have time to figure this out obviously but has this happened to anyone else? I am so excited and nervous. I figured she would come early anyways but to be told that was like WOAH.