Accumulating studies have shown that ELF interaction can be multilingual, especially through the use of code-switching (e.g. Mauranen 2014, Cogo 2017). Research has also found that ELF-focused interactions can develop into multilingual interactions over time (Kalocsai 2009, Smit 2010). However, studies have yet to examine how such changes occur between two speakers and how the changes can be observed at the discourse level. Longitudinal video recordings of conversations between two participants were analysed using conversation analysis and the framework of epistemics. Detailed analyses of word search sequences demonstrated a gradual change in the speakers’ self-and-other positioning of knowing a code-switched word. The data also displayed emergent use of an interlocutor’s L1 that was not used in earlier recordings. Findings suggest that non-English words can be gradually introduced into ELF interactions over time as speakers learn each other’s L1s, and the development is recognized by the interlocutors.