Claridge,

I don't know for sure whether updating your oversight would adversely affect your chances of getting into the FBI. But I suggest that you give serious consideration to withdrawing your application altogether and seeking employment elsewhere -- not because of your oversight regarding your most recent marijuana usage, but because of the high risk of a false positive on the polygraph and the serious consequences thereof.

About 50% of those who pass Phase 2 are being branded as liars by the polygraph. Given what is known about the accuracy of the polygraph -- the National Academy of Sciences has confirmed that it's junk science -- it is clear that many truthful applicants are being falsely branded as liars.

FBI polygraph results are entered into applicants' FBI HQ file. This is a permanent record that will follow you the rest of your life. Having a failed FBI polygraph on your record will make getting employment with any federal agency that requires a polygraph (or even just a security clearance without the polygraph) more difficult.

See my article, Just Say 'No' to FBI Polygraphs.