The first thing that would happen is everyone would need to figure out how much money they had in the new currency and how much it would buy. For the first few days there would be chaos as some people tried to take advantage of the situation and charge more than they should.

Until the new currency is fully distributed into the population there would be pockets where people were cut off from being able to buy goods and services because they don't have the currency yet. By the same token, there would be nationalistic groups that would create local economies based on the old currency and would resist using the new currency.

Currency trading would cease to exist because there would be no more fluctuation in value between currencies, but the investment money would all seek out either the highest interest rates or the most stable governments. Risk trading would probably become a new market.

After everyone got used to the new form of exchange, you would find that wages for same work would even out, with differentials based on skill and ease of delivery. People working in remote areas would be paid less than people working near the eventual user of the goods. However, it would also mean that the jobs that have been outsourced overseas would likely be repatriated. Manufacturing would return to the US.

Price differentials would be based on trust, quality, ease of transaction, dependability of service, and other 'user experience' issues. Poorly made products would be very inexpensive and well-made products would cost relatively more.

I don't think there would be "mayhem" so to speak, economy wise.

The "mayhem" would happen in the time it take for people to figure out the value of their new currency.. A good example is with the Euro. Everyone would have to get their calculator out to figure out how much things cost and get an idea if what they see is expensive or cheap.... If universal currency would suddenly happen, expect confusion and a lot of exploitation of this confusion by some merchants... however, the economy would stay unchanged.... that is, as long as a clear definition of the value of this currency is established.

if there is no value given, then yeah, it's total mayhem.

The problem with switching to a universal currency is the disparity between the various economies of each member country. That is why the Euro has baseline qualifications for countries to meet before they can start using the currency. Another example would be the joining of East and West Germany at the end of the Cold War. The West was a much stronger economy and the East and as a result, the East created a drag on the new country. Lesser Developed Countries do not have the skilled work force or infrastructure needed to support the high wages paid in the developed world. Furthermore, prices will be thrown off as suddenly products in Lesser Developed Countries cost much more. You would see periods of extreme inflation and deflation unless you started fixing prices, which is never a good idea because it constricts the growth of the economy.

Having regional currencies makes some sense (like the Euro) because it allows countries to function as a larger economic bloc. But a global currency provides few, if any, advantages.

My educated reasoning would be mass chaos. I have included some sources to illustrate times in history where a sudden change in currency led to vast difficulties.

I thought we already did ?

Well, whatever force led to " if overnight there became one universal currency" would also dictate whether or not there was mayhem. Anything powerful enough to affect that change could also probably quell disturbances, or accelerate them, depending.