Consistency criteria adopted for the management of persistent replicated objects in a distributed system define the degree of concurrency allowed among operations accessing objects. Several notions of consistency are known from the literature, among them are causal consistency, causal serializability, and serializability. In this paper, we propose a generalizing algorithm for concurrency control in a transaction system that exhibits a clean separation between policy and mechanism. A consistency criterion selected is manifested as a set of rules forming the policy. The mechanism, however, remains unchanged regardless of the currently used policy. The mechanism implements causally consistent message delivery and uses tokens and quorums of tokens to enforce access operation ordering according to the specified consistency criterion. Since a policy is implemented as a set of rules, switching on-the-fly from one consistency criterion to another one can easily be done whenever changes in access patterns or cost/availability requirements suggest a modification. An example of an application exploiting the advantages of switching among various consistency criteria concludes the paper.