Write the rule in one paragraph
State who keeps the old price, what ends it and whether it ever expires. Every agent should be able to repeat it without checking.
List the edge cases before customers find them
Cover upgrades, downgrades, a lapsed card, a reactivation and a second seat. Decide each one and note the answer in your reply templates.
Tell affected customers directly
Email grandfathered customers with their price, the rule and a date. A line in the changelog is not notice.
Keep a record on the account
Tag or note each grandfathered account so support and billing agree. Mismatched records are the main source of angry tickets.
Review the rule after a quarter
Count tickets about it and look at what confused people. Change the wording first, the rule second.