This is a strangely worded question. LuaJIT is compatible with PUC Lua 5.1. It will thus not run (without problems/tweaking) run code that is designed for Lua >= 5.2. However, the differences are not really big in practice. It is rather more likely that the new options (especially the FFI) that come with LuaJIT will lead to the most work when switching over. Not that they would be absolutely needed, but it just makes it all so much smoother and faster - which is probably the reason to switch to LuaJIT in the first place.