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GitHub User: thomchop
Simply updating the 'zurb-foundation' gem will not overwrite 'foundation_and_overrides.scss' or any of the other files in your assets folder.
However, if you then proceed to run rails generate foundation:install again, it will overwrite 'foundation_and_overrides.scss' (but not without prompting you for confirmation first). That generator will also modify 'application.css', 'application.js', and 'application.html.erb'.
rails generate foundation:install