item_price_ids already applicable for item_type plan cannot be removed
The plan's applicability cannot be changed on coupons after they have been applied to plans. For example, if you selected the coupon to be applicable on Plan A and a customer has now created a subscription with Plan A and applied the coupon, you will not be able to change the plan to which the coupon is applicable.
As a solution to the above hurdle, you can clone the coupon into a new one and archive the old coupon. This means that any existing subscriptions that have this coupon will continue to have it applied.
And the new cloned coupon can be changed to be applied to specific plans.
Note: The ID of the cloned coupon cannot be the same as the existing coupon, at least one character has to be different