You should notify all your affiliate partner in a form of email or popup message when they open their affiliate account since this kind changes is important to those small time affiliate user that relying on no minimum limit for their withdrawal.

I remember same case like this about not informing user for update of ToS from different casino that later on become an issue since the user can’t withdraw funds due to the new update.

This statement confirms to my recent inquiry that this update on minimum withdrawal is just newly implemented after the OP earns his commission.