[Hareeba!]

One free withdrawal. And a notice that any future withdrawals shall have a $40 fee. If the player wants to take all of his funds out with one withdrawal as a form of protest, so be it. Otherwise, one freebie, then the new rule takes effect.

Yes, that is the fair way to introduce the new fee and to treat your existing clients.
Perhaps also a second free withdrawal if there are futures pending.

[Scooter]

the book should have allowed a reasonable time, say 90 days for all existing clients to withdraw as much as they want before applying the new fee

This.

One free withdrawal only is inadequate.

The player should have the opportunity to withdraw all his funds without a charge, within a reasonable time period after notice is given both on the site and by individual emails.