is this something that, at the federal level, the president can simply stop enforcing? without needing any legislative changes (although they would be required to make permanent changes)?
Since there is no Constitutional authority to carry out "the war on drugs", it would be easy to stop. Even prohibitionists recognized that an amendment to the Constitution was required for Prohibition.
I believe the POTUSA could reschedule marijuana with an executive order, and if Obama for instance were to do so and reschedule it a schedule 2, all the medical marijuana dispensaries would be federally legal and the industry would be taken out of the gray area.