The current tenant with preferential rent is allowed to keep that rent, and its normal increases, for the time they are tenants, but they must acknowledge that it is a Preferential rent, and the LL continues registering the two: Legal and Preferential. When the tenant leaves, the new tenant is subject to the Legal rent that has been established for the prior time. It is not illegal for the LL to give a lease and Rider with the Preferential Rent listed. It is in fact provided for by the DHCR. Raising the rent to the legal rate is currently unenforceable FOR THE TENANT IN RESIDENCE. Not for subsequent tenants.