Purpose: This study aims to examine the impact of the transformation of an assessment on students' performance and perspectives in an English for Academic Purposes course in Hong Kong. The assessment was changed from the traditional pen-and-paper mode to an unproctored online mode. Design/methodology/approach: Using mixed methods, the research team analysed the differences between the assessment performances of those who took the course before the pandemic (n = 664) and those who took it during the pandemic (n = 702). Furthermore, focus group interviews were conducted with seven students regarding their perspectives on the unproctored assessment. Findings: The results revealed that, although there were no major differences in the overall grades of the two groups, students who were assessed online during the pandemic performed significantly better in terms of their English use. Nevertheless, the shift to online assessment had several negative effects on the students. Originality/value: Previous studies on unproctored online assessments (UOA) were concerned with potential learning quality issues, such as plagiarism and grade inflation. This study, however, provided empirical evidence that high-quality assessment delivery can be provided via UOA if the question types and assessment arrangements are carefully decided. Copyright © 2023 Emerald.