The role of domestic tourism as a substitute for international tourism has not received adequate attention in the literature. However, the potential for substitution has become particularly important in the COVID-19 pandemic context which has significantly impacted travel flows as well as the urgent need to reduce greenhouse gas emissions. Drawing upon data on major tourism destinations and generating markets, a tourism policy thought experiment is conducted to explore the substitutability of domestic for international tourism in selected countries in light of COVID-19 and other situations, such as the climate crisis and the urgent need for low carbon tourism. The analysis and discussion highlight the complexities in achieving sustainable substitution in rescaling international mobilities to domestic. It is argued that without careful changes to overall tourism provision and consumption behaviours in the international-domestic tourism division, a (partial) shift may provide short gains but is likely to fail in the long term. The paper concludes with a critical analysis of contemporary debates on COVID-19 related tourism transformation in relation to substitution between domestic and international tourism and sustainable tourism futures.