It's not so much them switching to a gold standard or even devaluing their currency but the fact that they want to dethrone the US as the reserve currency. However that won't occur until the US dollar is no longer the absolute currency for which OPEC oil is sold under. It is the petrodollar relationship to that insures the US remains "The World's Reserve Currency" of choice because ME oil sales dominated sole in US dollars is what fuels demand for our currency world wide. It is the mechanism that allows us to run up debt and export away all the associated inflation which would occur US currency were not so intrinsically tied to oil sales in the ME.
https://www.youtube.com/watch?v=PCAFnCo6nyM
That's why the Iran deal is important for the U.S to bring Iran into the fold and maintain demand for the dollar internationally.
Kissinger flew over to Saudi Arabia in 1973 to get Aramco to sell exclusively in dollars in exchange for U.S. military protection, and by 1975 all OPEC nations followed suit and exclusively traded in the dollar. However anyone in the late 60s and early 70s remembered when OPEC wasn't exactly America's friend.
With the political split between US and Saudi Arabia that began over America's unwillingness to go to war and get rid of Assad, the US knows if the Iran deal falls apart, Saudi Arabia's willingness to go along with the petrodollar is not going to go on indefinitely, and OPEC will likely follow whatever direction the Saudis take, so the long term prospects of the status quo isn't good for the dollar.