This paper examines an area of ongoing change in English — deontic modality — and uses an archive of synchronic dialect data from England, Scotland and Northern Ireland to discover new information about its development. History records a cline in this system from must to have to to have got to. By taking a cross-dialectal perspective and utilizing comparative sociolinguistic methods we present a possible reconstruction of the later steps in this process. The results reveal dialectal contrasts in the proportion of older and newer forms, but similar patterns of use. Must is obsolescent and there is an unanticipated resurgence of have to alongside pan-dialectal grammatical reorganization: (1) have to is being used in contexts traditionally encoded by must and (2) have got to is specializing for indefinite reference. Young women are the leading edge in these developments suggesting that systemic adjustments in grammar combine with sociolinguistic influences to advance linguistic change.