Will the Single Convention on Narcotic Drugs (our treaty through the UN), prevent the US from making this change, or ultimately legalizing marijuana in the US?

No, for three reasons. First, the President’s directive implies that treaty considerations shouldn’t determine marijuana’s scheduling, as he requested the reconsideration of its status. Second, the U.S. is already non-compliant and would remain non-compliant with the Single Convention due to state-regulated adult-use markets, even if marijuana stays in schedule I. Third, the Single Convention allows non-compliance if it conflicts with the signatory state’s constitutional framework, which applies due to the need to destroy the regulated state-level industry for compliance. The federal government would need to do so  in 3 out of 4 states, many of which adopted legalization through their own constitutional amendments.