This article reports the preliminary findings of a study examining the semantics of modal verbs in heritage Ambon Malay, a language variety spoken by Dutch-Ambon Malay bilinguals in the Netherlands whose dominant language is Dutch. In this study, I examined the use of the necessity modal musti [must] in the speech of heritage language (HL) speakers and compared it to that of monolingual homeland Ambon Malay speakers and monolingual Dutch speakers. The findings show convergence between the modal system of the heritage language (Ambon Malay) and that of the dominant language (Dutch). More precisely, the heritage necessity modal musti [must] has extended its semantic range to resemble its Dutch equivalent moeten [must.] I discuss three main factors that account for this innovation, namely (i) psychological factors – semantic convergence is one of the strategies adopted by bilinguals to reduce their cognitive load, (ii) universal principles of language development in contact settings ̶ conceptual naturalness facilitates semantic influence from Dutch, and (iii) social factors ̶ the language history of HL speakers shows that the innovation correlates with type of bilingualism and amount of exposure to Ambon Malay. Finally, the findings of this study support the Functional Discourse Grammar hierarchy of language change and, to a lesser extent, the typological hierarchy of Matras (2007).