On the legal side, I expect that this will depend quite strongly on what precise kind of open-access policies they will implement. If "open access" means just "no payment to read" (but all reproduction rights remain reserved), then I suppose they could simply change the policy again at a later point in time. On the other hand, if "open access" means that articles are released under e.g. a Creative Commons license, then anyone can upload those articles to repositories that will preserve them indefinitely, and since the CC license once granted cannot be revoked, there would be no legal way of suppressing the free distribution of those articles in the future.

(Caveat: I'm not a lawyer, and this is not legal advice.)

On the technical side, I don't see how there could be a viable guarantee that access isn't restricted at a later time.

So in summary: in my opinion, "open access" is only truly open access if the contents are licensed appropriately to enable free distribution.