If your tenancy is sold and you get a new landlord, the new landlord cannot require that you sign a new lease. You are a part of the sales trade and that’s why your original lease is still valid. The responsibility for familiarize what type of lease you have as a tenant is on your new landlord because your rights and obligations are the same even though the tenancy got a new owner. Therefore you are not obligated to sign a new lease, though it can be a good idea to consider a new lease depending on what is says about your rights and obligations.