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That person couldn't go back and fix things, because the records would show that there was one statement made one time and another statement made another time. That, by definition, means that one of the statements is a false official statement, which is a violation of the UCMJ.
Now, it is unlikely that the person would go to jail. While it is a criminal violation, most such violations are handled through administrative separation from the service for fraudulent entry. Regardless, there isn't really a way to go back and fix the matter at this point.
Either the test comes back positive, which would result in an administrative separation for drug use (and perhaps an Article 15 as well) or the test comes back negative, at which point the individual wouldn't face any trouble at all here. So, unfortunately, the best option is for such an individual to sit on their hands and wait, hoping the test result is negative. Going forward now can't resolve the issue any more favorably.
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