But then any precompiled unit (in fpc) which uses one of those units, still uses the old code.
If you want to patch fpc, then by all means change the source, rebuild and re-install.
If you have the sources then
cd /path/to/fpc/sources
make all
make install
On Windows, this will build fpc, and then install it in c:\pp.
You'll have to generate a fpc.cfg file for this patched compiler, for which you can use fpcmkcfg.exe (it is in the same folder as your new fpc.exe).
Also, doing it this way will prevent name clashes and the infamous "unit not found" error (when it finds an incompatible ppu).
Bart