yes, 64bits should works.
Maybe there is a downgrade command using pacman.
I did manually, downloaded all file with "5.4.0-1" on it, from
https://sourceforge.net/projects/msys2/files/REPOS/MINGW/i686/mingw-w64-i686-gcc-5.4.0-1-any.pkg.tar.xz
mingw-w64-i686-gcc-ada-5.4.0-1-any.pkg.tar.xz
mingw-w64-i686-gcc-fortran-5.4.0-1-any.pkg.tar.xz
mingw-w64-i686-gcc-libgfortran-5.4.0-1-any.pkg.tar.xz
mingw-w64-i686-gcc-libs-5.4.0-1-any.pkg.tar.xz
mingw-w64-i686-gcc-objc-5.4.0-1-any.pkg.tar.xz
and installed with pacman.