The hardest thing to change after the program is already finished is getting rid of allocations. But other than that MISRA compliance is not that difficult and avoiding allocations is not that difficult either if you plan for it from the beginning.
If you know what you are doing it is not difficult, but if you are doing it for the first time it a good tool to learn how to think about all the repercussions of how you program something. MISRA isn't really to catch problems with c it is to catch problems with people using c. So knowing what those problems are and how to avoid them is incredibly useful.
