Good question.  Simply Fortran will work fine with just about any Fortran compiler, so you should be able to upgrade to the 4.7 branch of GNU Fortran.  However, there are some caveats in doing so:

1. Simply Fortran does assume you're working with GNU Fortran, meaning it expects certain compiler flags to work in certain ways.  Changing compilers could cause some compiler flags to not work properly, although the chances of this happening are slim.

2. More specifically, Simply Fortran, for in-editor syntax checking, assumes a specific error and warning format to be issued by the compiler.  Simply Fortran's IDE has only been tested with the version of GNU Fortran with which it ships.

Approximatrix attempts to keep up to date with the official MinGW version of GNU Fortran.

Is it safe for Simply Fortran users to update the files found in the Simply Fortran directories with the corresponding files found in the latest GNU releases?  I ask, because GFortran, for example, is now on version 4.7.

I don't see this as important because, in Simply Fortran 1.17, I have not come across any difficulties that make me think I might need a later version of GFortran.  I'm just curious about the answer.   
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John