Yes, online casinos can change the payout on slot machines. Many times it is really easy for the casinos to do so by changing the pressure on the reels or by changing a chip. This is how a casino cheats on its players. However, this is not possible in progressive slot casinos. This is because, in progressive slot casinos, the payouts are not under the control of casinos. Here the payouts depend on the wagers made by the players. Thus, the casino cannot decide upon the payout transacted through slot machines.