You could advise the lab but like everyone has said, 1/2 stop is not that big of a deal with color negatives. Next time you shoot a roll, bracket an exposure with 1/2 over, normal and 1/2 under just to see the actual difference.
And as far as forgetting to change the ASA, don't feel like your alone.
We've all done it and I used to be so bad about forgetting that about 30 years ago I took an old labelmaker and made a little strip to go on the back of my Spotmatic II field case. It said, "Set ASA" and it's still there!
CW