In discounts, how can someone change the discount apply method without deleting the discount?
I’m not aware of a current way to accomplish this if the Discount Code has been used. Once a Discount is linked to a Discount Profile AND used on a Sales Order. It is no longer able to be modified.
If the Discount has not been used on a Sales Order but has a Discount Profile you can delete the profile and that will allow the Discount Apply Method to be modified.
Can a enhancement request be submitted to make the profile inactive so that it stops applying?
You can always submit an enhancement request. Though I think there would be hesitation on maintaining the integrity of the old or in flight records if the Apply Method changes underneath. Not an unsolvable problem but may be more complex than what it seems at first glance. I think the main question would be on why a new Discount Code is less preferred. Everyone has their reasons for things. I might include that in the request though.
I am going to add an effective from date as a date in the past.
Let us know if your effective date transition is helpful. Otherwise, I would like to better understand if the need is coming from incorrect initial set up of the discount, or if there is a business need to swap the apply method.