This study investigated whether L2 English learners' prosodic properties in speech acts change as they are immersed in the English-speaking academic community over time, and if so, what individual and contextual factors (proficiency, orientation to language study, and target language contact) potentially affect these changes. Forty-seven Japanese learners of English in an English-medium university in Japan completed a speaking task that elicited two speech acts (request and opinion) three times over one academic year (8 months). Their speech was analyzed for discourse intonational features (e.g., tone choices, prominence ratio, and pitch range). Results showed that all prosodic properties changed over time, although the pace and patterns of changes differed among the properties. Proficiency and language contact significantly affected the change in tone choice, but no other relationship was found between individual/contextual factors and changes in prominence ratio or pitch range.