I recently migrated from Lua 5.2 to 5.3. In my case, I had to do nothing special in my scripts (they are running fine without changes) but had to adapt my code for the differences in the C API. As I use Delphi in my "host executable" (Win32, Android-ARMv7 and MACOSX-x86), it was needed to recompile LUA 5.3 in different compilers and adapt the "host code" for the new number type (Integer).
For the LUA scripts, my perception is that there are very small changes to do. For sample, some functions in the Math Library (like math.atan2) were removed (but there are ways to overcome this). The new behaviour of numbers (sometimes they are Integers, sometimes they are floating point numbers) seems to be compatible with old codes.
