You have just gone through a stock transfer and your questions are highly relevant. As your property has been transferred to a housing association, as a matter of law you ceased to be a secure tenant from the day that the transfer took place. You are now an assured tenant. There is nothing to be gained by not signing the new tenancy agreement which is more likely than not to contain your most important rights like right to buy, succession, consultation, assignment and so on. So check the document carefully. The key advantage for signing is that the housing association has signed a legally binding agreement with the council that it would abide by the terms of the new enhanced assured tenancy agreement. So the housing association will respect the preserved rights in the new tenancy agreement.