The question you asked is “what if Congress changes the law?” The answer truly depends on how the law is changed. Let’s say Congress passed a law that reclassified marijuana from a Schedule I drug to a schedule V drug. It’s still a controlled substance. Way too many variations to address them all.
Well…..drug and alcohol (broadly, substance) abuse or dependence is the behavior the administrator is required by law to control for. Substance abuse and substance dependence fall in the category of mental disorders as the medical condition. There’s also restrictions on consumption of alcohol prior to exercising certificate privileges and prohibited impairment levels.
Next, it’s generally accepted marijuana contains THC, and THC is a psychotropic substance, there’s also that hurdle to jump as well. The entire class of medications is not issuable by an AME and must go to the administrator for decisioning, at which time the underlying condition or need for use is what will be looked at to determine the aeromedical impact.
So I don’t think it’s likely the administrator is going to have any effective changes to their stance on marijuana were congress to take any legislative action on that subject.