Which u/s did they change your dates at - 12 weeks or 18-20 weeks?
The earlier scans are usually more accurate, becuase a lter scan can just show you are having a smaller or larger baby than the average size.

You are not "overdue" until 42 weeks, so if you like you could go on the earlier dates and request a scan for placenta function when you would be 41-42 weeks by those dates.

You can be sure enough of your LMP dates, but thats only really of use if you o'd on day 14. If you have a regular cycle thats great but if not then its your O date or conception date that you'd need to know to be more accurate.

I hope ive been more helpful than confusing..