All you need to do is recompile. Rosetta will compile the MPI version into a different, independent location in your build folder and add “mpi” to the executables in the bin/ folder so you know which to use. The old, single-threaded ones will still work, and say “default” instead of “mpi”. For example, you’ll have fixbb.default.linuxgccrelease, and fixbb.mpi.linuxgccrelease. (The version with no “default” or “mpi” is whichever was built most recently).