I have to agree with this... If you make substitutions in a stable band setup, you owe it to the management to inform them in advance of this happening (unless the case of a sudden illness or emergency), and offer them the option to approve the change, or to hire alternate entertainment for that night (I really doubt that, though!), and at least voice their concern. If any adjustment in scale is suggested by management at that point, the act has the option to just take the night off, or accept the lower scale.

My band usually has someone going somewhere every now and again, but a month in advance is usually asked for and given.