Change Template from a Future Date

Changing the length of appointments from 15mins to 20mins from a future date would be a great feature ratehr than having to change it from now.
If a person who has a 15 min appointment cancels, and then you go and book a new appointment in that spot, it won’t work as the new length of the appointment is now 20mins. (it doesn’t fit into the 15min gap now created)