Imagine I buy and sell coffee products, and have done so legally for many years. However, the FDA decides that coffee is a carcinogen, an addictive drug, and extremely dangerous, and consequently places it on Schedule I drug, along with Heroin.
I continue to buy and sell coffee entirely unaware of the change in law. Then the feds come over, bust down my door and I am facing a charge of dealing schedule 1 drugs, and life imprisonment.
What is the legal situation here? Plainly I had no mens rea, I was entirely unaware of the change in the law, and reasonably would have expected my previously legal activity to continue to be legal.
However, I also know the epithet that ignorance of the law is no defense. So what would my legal situation be here?