Can exchanges change the rules any time they want to?

​The answer is yes. Such an event happened at the Comex a number of years ago!

It was called the "Great Silver Fiasco," which was associated with the Hunt brothers.
Silver made 19 consecutive moves to the downside, and destroyed several individual commodity traders, when the Comex Board of Directors changed the exchange rules to allow silver prices to fall and not rise. A "liquidation only" restriction was passed by statute, after the Comex had raised margin rates to drive the small traders from the market. The effect of this statute meant no new long contract positions could be initiated. The Comex Board of Directors were allegedly short forty million ounces of silver, not a conflict of interest according to them. Their dictatorial powers allegedly gained $400,000,000 for each $10 silver decline.

The Hunt brothers had profitably speculated on silver, until the Comex Board of Directors changed the rules. The Hunts were wiped out – at least as far as their silver holdings were concerned. They had been trying to corner the market. Instead, they were left holding an empty bag. They had been steadily buying silver and so were dollar averaged at increasingly higher prices. They didn't mind that, because everything they had purchased at lower prices was profitable as prices rose. When the Comex Board refused to let prices rise even higher and allow them only to fall, they eventually fell below the price where the Hunts had made their initial purchases. The intervention by the Comex Board was a massive example of market manipulation by insiders, and an excellent example of intervention in what had been a free market. There are no longer any free markets of any consequence in the U.S. When the initial Plunge Protection Team intervened to save the stock market in 1987, it was the end of free market capitalism in the U.S.

Lesson: Exit markets when exchanges change the rules unless the rules work to your favor, or you are a member of the Comex Board of Directors.