This paper aims to explore sound change within the framework of Optimality Theory. Sound change is a complex phenomenon with various facets of linguistic operations interleaved. On a larger scale, sound change is a grammar change which makes a crucial reference to the system as a whole. This paper gives an Optimality-theoretic account of the diachronic change attested in phoneme inventories of Old and Middle English. We argue for the inclusion of symmetry as an essential constraint for the change in phonological systems. With regard to reranking, we demonstrate that sound change is the promotion of the structural and markedness constraints over the faithfulness constraints.