Can I change or cancel my plan?
Yes, any time, from Billing and plan. A cancelled plan keeps working until the end of the period you paid for.
Yes. An owner changes plan under Settings, Billing and plan by pressing the button on the plan they want.
To stop paying, press Cancel plan on the same page. Everything keeps working until the end of the period you have paid for. After that the app stops until a plan is chosen. Nothing is deleted, and an owner can still download everything.