A very similar proposal came before the regional Presbyteries several years ago—I’m thinking around 2005 or 2006—and failed to be ratified by a majority.
Still wondering—50% or 2/3 majority of Presbyteries?
50% or 2/3 vote to pass within each Presbytery?
It really should not matter, because Genesis 2/Matthew 19 should not be subjected to a vote.
Simple majority (50% + 1) vote of the presbyteries will adopt the change.