If a coach changes the price of a program it affects all new sign ups plus all existing clients on their next payment date.
However, a coach can create a coupon in order to discount and so maintain the current price for those existing clients they wish to do so for.
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In addition, though not necessary because the code doesn't need to be given out publicly, a coach can select as one of the coupon creation options 'x number of uses' where x equals the number of clients it's being created for.