Here is one way:
Step 1: Format the columns you want to change as General (or something other than Text)
Step 2: Highlight the columns you want to change. Do a Search and Replace (from the Edit drop down menu, selecting Replace), replacing the equal sign (=) with the equals sign (=).
This will work as long as all your formulas start with an equal sign. If they start with a plus sign (+), do a Search and Replace on that instead.