Just contacted Openrent support directly and they confirmed the following so hopefully this is of use to other Welsh landlords who have used Rent Now to set up their OCs:
The changes come in to force from 1 June 2026 and a notice stating that new terms to the Occupation Contract apply must be sent within 14 days of this date. OpenRent will be sending the relevant notice to exisiting tenants in Occupation Contracts on the landlord’s behalf. We will also be updating our standard Occupation Contract so that any new contracts signed contain the updated terms.