Sex can't change from conception. However, human error is well documented. At worst the technician was wrong and you'll see that at the 20 week ultrasound. Or the initial prediction will be proved true. That's why I like to wait to see penis or no penis. Haha.
The sex of the baby is determined by the sperm. If ur specialist found out through blood test that gender is a girl, then it must be so
I did the same and on the 10th go in for the anatomy scan as well. My Dr said its for the parents to see the baby and get the measurements of the baby. They compare it with the further along anatomy scan they do not long before your due date to determain if you need to induce or allow the baby to decide and all that good stuff. Never heard of the sex being different though