Change case of text not identified by a syntax item?

I would like to change the case of all the text in a file that isn't
identifed by any current syntax item.

Text not identified by "syntax/fortran.vim" is for the most part
variables, program unit names, etc..

So this would simply allow me change the case of all my variable
names.

I've been racking my brain to figure out how this could be done to no
avail.

I could probably just write a perl script with some big hairy regex to
accomplish this but vim syntax highlighting already does such a good
job of identifying everything but variables it would be nice if I
could make use of it.

Any suggestions or nudges in the right direction would be appreciated.

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