Only the US has to agree to the proposal.
Allison Don 14, has published as part of University of Minnesota Law School Scholarship Repository, 2014, "Lighten Up – Amending the Single Convention on Narcotic Drugs," Minnesota Journal of International Law, https://scholarship.law.umn.edu/cgi/viewcontent.cgi?article=1278&context=mjil, anita
With 153 current parties to the Single Convention, arriving at a consensus may prove difficult. This does not preclude the option to amend as “amendments require agreement between treaty parties, but not necessarily between all parties.” Once an amendment has been proposed and adopted, parties are free to decide if they will become a party to the amendment.159 Those who opt not to join the amendment remain bound by the treaty’s original obligations.160 By proposing an amendment that would permit the use of marijuana for recreational purposes, those countries who wish to pursue such legislation would be permitted to do so and those countries who remain in opposition would be able to remain parties to the original treaty preventing the use of recreational marijuana.