As far as I know (though looking over the Winter '18 release notes, which mentions that this critical update will be enabled for everyone as part of that release), the critical update you mention is only to allow you to set up CSRF protection on GET requests on a page-by-page basis (It shouldn't automatically turn that on for any existing page).
That option will likely only appear on Visualforce pages that have an API version >= 27.0, and I highly doubt that Salesforce would go and automatically change API versions of existing Visualforce pages (I'd imagine that would be at least as big of an undertaking as phasing-out TLS 1.0 was, if not bigger).
As usual, it would be prudent to enable this in a sandbox environment first so you can figure out exactly what it changes (and if it breaks existing code/pages/integrations/etc...)