Account Downgrade
Send this when an account moves to a lower plan, whether the customer chose it or a payment lapsed. It shows the previous and current plan side by side, lists what changes and what stays the same, and links to a plan comparison. Replace the plan names, date and limits with the recipient's real values before connecting it to your billing event.
Loops
Be exact about what changed
Pass the previous plan, current plan and effective date as required transactional data, and list only the limits that actually apply on the new plan. Say that data and settings are kept only if your product keeps them. If the downgrade followed a failed payment, send the Payment Issue template first so this email is not a surprise.