Standardized and transparent priority setting in medicine, desirable as it is, will generally exacerbate inter-temporal equity problems arising from changes in treatment priorities: when can it be fair that the treatment of already waiting patients who would have had priority under an established system should be postponed (withheld) for an extended period of time to advance the treatment of others under a reformed system? The reform of the Eurotransplant system of priority setting in kidney allocation (ETKAS), which is in many respects ideal, is a case in point. To give due weight to new medical knowledge, waiting time after the onset of end state renal failure should change from a priority-enhancing to a priority-reducing factor. Since those who have gained in priority by waiting under the present system would be set back under the new, severe problems of transitional justice must be overcome when responding to advances in medical knowledge. The paper explores conceptually some possible ways of rule change and indicates their general relevance from an ethical and a practical point of view for future problems of medical resource allocation under transparent, standardized priority-setting rules.