What you're doing here is managing someone's expectations of enforcement of a license by you, the copyright holder. You're not actually changing the license under which people can get it from the site.
If you have this in your profile:
All of the code I've written and contributed to Stack Overflow is dedicated to the public domain, or (at your option) available under the terms of CC0
Then what you're doing is telling people you have positively no intention of enforcing CC-BY-SA. That's a nice thing to do, and I won't caution you not to do it, but there is a caveat.
The law would treat [your promise not to enforce the stricter license] like a gift, and gifts can be rescinded. This makes project managers nervous. If you really want to legal-proof it, then you'd have to publish the code somewhere else (e.g. one giant git repo with all your stuff in it, under the terms of CC0). As long as your code exists somewhere else, explicitly licensed under CC0, then you remove the whole gift aspect of it. Just point to that repo in your profile.
I agree that it's ridiculous, but that's .. just the way the law works. This is why in the new license scheme we've been working on, we have to ask for something in order to grant the option to not carry the MIT - or it's a gift. That works out easily because we ask for attribution.