Or the federal government can just use it as a means to arrest pot-smoking undesirables at will? I don't really see how state laws come to a head with federal ones in this case - state police and lower will not enforce the federal laws, but FBI/DEA etc. can and probably will continue to. When it's a federal law granting someone a right (e.g. Roe v. Wade, the 14th amendment) then the federal government gets to go in and change state laws, but I've never heard of a case of federal laws changing because something it criminalizes was explicitly legalized in state laws. The most analogous historical precedent we have is prohibition, and iirc state laws didn't change until after the ratification of the 21st amendment, so that doesn't really apply. Unless you're saying that states would pass a constitutional amendment, which seems a little far fetched to me, though not impossible.