Meal-count changes apply from your next delivery, not the current one. If you've already passed the cut-off for the upcoming delivery, your current count stays for that one and the new count kicks in from the delivery after. Your account shows the price preview and the effective date before you confirm.
Your account updates immediately and you'll see the change reflected. Behind the scenes, the change can take up to 48 hours to fully process. Important: if a delivery has already passed its three-day cut-off, that delivery will still be sent - pauses and skips only stop deliveries that haven't been locked in yet.