Partly. Weeks and Buffer Days to Charge Before Renewal are read from the plan every time the renewal job checks which subscriptions are due, so changing them affects the next renewal of existing subscribers as well as new ones. Billing Renew Day is copied onto each subscription at the moment it's created, so changing it on the plan only affects subscriptions created afterward - it won't move the renewal day of subscribers who already signed up. Allowed Start Days are checked only when a customer is starting a new subscription, so changing them doesn't affect anyone already subscribed. Price, delivery-day, category and portion changes made on a price row are only pushed to existing subscribers when you choose an "...and active subscriptions" option in the confirmation pop-up that appears after you save that row.