This is a good article from the ABA’s Business Law Today publication: Online Contracts: We May Modify These Terms at Any Time, Right?
In short, no, it doesn’t reliably work to tell customers or website visitors that you can unilaterally amend their contracts, just by posting a new version. You need notice and consent, as The Tech Contracts Handbook points out, in Chapter III.S. (See the sample language proposed for unilateral amendment at this site, which of course comes from that chapter. The language works for customer/vendor relationships, not for visitor/website agreements.)
If you want to save time with the article, scroll down to the four bullet-point suggestions, near the end.
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© 2017 by David W. Tollen. All rights reserved.