It would be fair from them to say : " from now on the percentage on your affiliate will be reduced to xx% " but they should maintain untouched the one they advertised originally because that was the agreement on which it was born and the agreement had taken hold.
Exactly right.
They can change the rate on "new" players referred after some announced future date -- giving a heads up to affiliates that such a change is coming, so they can decide whether to continue promoting them under the new plan. But lowering the rate on players referred before that date is not acceptable. This is flat out theft -- very unfair, very unethical.