to find-and-replace within a formula

Numbers 3.5.3

OS X Yosemite 10.10.5

Hi all: My monthly payroll processing is hobbled by the current strain of Numbers not providing a search-and-replace function that applies to formulas.

Here's a typical formula from my payroll spreadsheet:

=SUM(E5,July 2015::'pay stub - Marvin'::F5)

Now that it's July, I need to change each of approximately 60 within-formula instances of July 2015 in my payroll spreadsheet to August 2015.

Is this something apple scriptable or Automator-able? Alternatively, could I use a variable where "July 2015" sits in each formula and enter that variable's value one time per month (each payroll month is its own sheet) and update each month that way?

Up to this point I've been exporting to Numbers '09, applying search-and-replace-within-formula there, then returning the file to modern Numbers 3. It'd be great to improve my process and obviate these extra steps.

Thank you.