Language acquisition is a well-established avenue for language change (Labov, 2007). Given the theoretical importance of language acquisition to language change, it is all the more important to formulate clear theories of transmission-based change. In this paper, we provide a simulation method designed to test the plausibility of different possible transmission-based changes, using the Tolerance Principle (Yang, 2016) to determine precise points at which different possible changes may become plausible for children acquiring language. We apply this method to a case study of a complex change currently in progress: the allophonic restructuring of /æ/ in Philadelphia English. Using this model, we are able to evaluate several competing explanations of the ongoing change and determine that the allophonic restructuring of /æ/ in Philadelphia English is mostly likely the result of children acquiring language from mixed dialect input, consisting of approximately 40% input from speakers with a nasal /æ/ split. We show that applying our simulation to a phonological change allows us to make precise quantitative predications about the progress of this change. Moreover, it forces us to reassess intuitively plausible hypotheses about language change, such as grammatical simplification, in a quantitative and independently motivated framework of acquisition.