Moved up my due date

I was 28 weeks until I went in for a 3rd trimester growth scan and they said he's measuring 30 weeks so he will most likely come closer to Sep 10th rather than Sep 24th. Has anyone had this happen and then get moved back to their original date later on? I'm just wondering if they'll keep moving my date now, or if this is pretty solid. Also, when they make determinations on whether I'm full term, which date will they use? I'm a bit confused how this works. TIA!