Congrats

and welcome!
Estimating dates from scans gets more unreliable the further along are you - so a 'dating' scan at approx 7 weeks is the most accurate.
I dated almost a week behind my LMP at my 7 week scan (which corresponded with what I knew of my long cycles), and then I was slightly ahead at the 12 week, and about 2-3 weeks ahead at my 20 week scan!
This baby is definitely measuring on the large side (as was my DD) - but in those scans, having a larger baby translates as being further along (as they are simply based on a set of population data).
Maybe your baby has just undergone a growth spurt, or changed position in your uterus, so you're measuring ahead? I wouldn't stress too much - there is a large degree of inaccuracy with the scan estimates!
Obviously another scan may offer some peace of mind in terms of size and fluid, but I don't know that they would change your EDD based on a late scan alone? I guess you may just have to be prepared that you may not end up with a specific EDD - it may be a range - and something you might need to discuss with your MW so that you have an agreed date to fall back on, should you go "over" and need to talk about induction etc?
(i.e. you probably wouldn't want to risk induction at an estimated 42 weeks, if you really were only 40 weeks, and your body just hadn't done it's thing yet...)
Good luck anyway, it sounds like baby will be here sometime in Jan

(come and visit us in the Jan thread!)