You would pretty much need to completely redo it I think. This would be different if there was a disassembly as you could basically tell the compiler to just output a 1.0 or 1.1 version as you please, but this isn't the case.
Revisions and versions of ROMs are two completely different entities from each other. If you compared one to the other in a hex editor you would see that the innards are not really the same at all. FFVI 1.0 has a superior version of sketch the fix square made to 1.1 in patch form (among the 100+ other bugfixes made for it), therefore people just happily use that.