The old-style build uses a set of shell scripts, whereas a configuration file contains the compiler flags and such. You can recognize the old-style build because $MIR/bin and $MIR/lib exist and has a few subdirectories. The new-style build works the other way around and places those normally inside a single $MIR/build directory.
% edit $MIRPROG/calib/mfcal.for % mirboss % mir.prog mfcal
If you want to change a subroutine, it will depend if your Unix version uses shared libraries if you need to recompile the relevant programs as well. Currently the answer is : Linux uses a shared library ($MIRLIB/libmir.so), whereas Mac does not.
% edit $MIRSUBS/uvdat.for % mirboss % mir.subs uvdat
Another common scenario is that code has been updated by the MIRIAD developers. In that case the procedure would be the conservative approach and recompile everything:
% cd $MIR % cvs update % mirboss % mir.install subs prog
Check the recent logfiles in $MIR/tmp for compilation or linking errors.