What happens to accounts (customers) that subscribe to a plan if I change the price or delete the plan?

Recurly supports price grandfathering. When a subscription plan changes, your existing subscribers will remain grandfathered in at the terms they agreed to. New subscribers will be subscribed with the updated pricing terms. Price grandfathering does not apply to changes in bill cycle (ie, monthly to yearly), however, so changes to billing frequency should be made with a new subscription plan.