That will never happen since SAR is pegged to the dollar and OPEC's transaction currency is US Dollar.
Nothing in the rule book or international law that says you can't unpegged! In fact we have seen it happened many a times. The reasons for the pegging the first place is ensure stability of prices. Accepting dollar for payment in oil has nothing to do with pegging, and more to do with politics when US coerced Saudis to accepting it for payment in return for the biggest cartel in the world.
Since they lose some of the money through currency exchange. With the SAR pegged to the dollar SA does not lose a penny through any transaction.
As for OPEC transaction, it's not dominated it's the rule since the price is pegged to the two major oil exchange markets.
Not ture. Pegging does not prevent the loss of money! It reduces the loss by taking out the currency risks, but it still incurred transaction action cost and holding cost!
Further from you;
Look in the later half of the 21st century, oil will not be a commodity any more since the hydrogen society will over take the present carbon society in which case the petro-dollar scheme will collapse.
OPEC nations as well as all nations dependent on exporting oil all feels this coming just not knowing when it will come.
So talking about RMB taking over the US dollar as a transaction currency is meaningless since there are no commodities that Mainland China has a monopoly of.
So you got a better crystal ball than any of us here then. Bully for you! But meanwhile In the real world, I quote J M Keynes:
'In the long run, we all be dead'!