>Most software, even that you bought, have a checkbox "I agree to the EULA". If you don't check it, you can't install. So, if you're running it, you agreed. For web sites, if you don't agree, you can't sign up. Makes it fairly simple to show you agreed.
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>>Not only would you have to be noified, the vendor would have to prove you accepted the original EULA's component that terms may change and that you now knowingly accepted the new EULA.

That only takes care of the initial agreement, it does not help with updates. Additionally, the legal enforceability of those checkboxes is not established. Of course, a person is still ethically bound, I am just speaking of legally.