Make sure that after you change the architecture of your project that you perform a "Clean" operation first by selecting "Clean" from the Build menu.  I'm guessing that the compiler is attempting to link your 32-bit object files that reference 32-bit runtime functions.

Yes, that solved the problem.  Thanks for the quick response.

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    "C:\Program Files (x86)\Simply Fortran 2\mingw-w64\bin\gfortran.exe" -o "test3.exe" "build\Test2.o" -LC:/PROGRA~2/SIMPLY~2/MINGW-~1/lib/
c:/program files (x86)/simply fortran 2/mingw-w64/bin/../lib/gcc/x86_64-w64-mingw32/4.8.2/../../../../x86_64-w64-mingw32/bin/ld.exe: i386 architecture of input file `build\Test2.o' is incompatible with i386:x86-64 output
build\Test2.o: In function `fddata_':
C:\Users\geoff\Documents\Simply Fortran test/./Test2.f90:629: undefined reference to `__gfortran_st_write'
C:\Users\geoff\Documents\Simply Fortran test/./Test2.f90:629: undefined reference to `__gfortran_transfer_integer_write'
C:\Users\geoff\Documents\Simply Fortran test/./Test2.f90:629: undefined reference to `__gfortran_st_write_done'
C:\Users\geoff\Documents\Simply Fortran test/./Test2.f90:630: undefined reference to `__gfortran_st_write'