I would still go by your original dates and if your carer has half a brain they will too. Up until 12 wks of gestation, the scans taken in that time are the most reliable for dating as until that point nearly all babies grow at the same rate - after that genetics takes over. Scans to estimate size at that late a stage are notoriously inaccurate and honestly there should be a law against them - after all they were only doing to scan to check the position of the placenta and not the size of the baby.

It could also be entirely possible that the baby could be a lot larger too, but there is nothing to worry about if baby is large this time - it still has to come out doesn't it?