This study analyses the production of French /y/ and /u/ by 42 native English learners of French (ELoF) at the start and end of a Residence Abroad (RA) in a French-speaking country. As an approximation of both phonological and phonetic development, categorical change is teased apart from gradient change using k-medoid clustering of acoustic data and different input measures are tested as predictors of both types of development. Results of the phonological analysis reveal that while no change occurs for /y/, the proportion that learners correctly use their back vowel in /u/ contexts increases over the RA. The quantity of French input declared over the RA is a significant predictor of this categorical change, especially the amount of auditory and visual engagement (e.g. listening and reading in French). Results of the phonetic analysis indicate, instead, that /y/ becomes more target-like over the RA, while /u/ makes less progress, suggesting a partial mismatch between the phonological and phonetic levels. Nevertheless, the phonetic development of /u/ is more substantial for individuals who: (1) have been learning French longer, (2) have yet to experience naturalistic exposure by the start of the RA, and (3) are English language teachers over the RA rather than students at a foreign institution. Taken together, these results have implications for the link between second language (L2) input and L2 production, the assumptions of L2 speech models, and the relationship between different levels of linguistic representation in L2 speech learning.