Every year I let my Premier membership expire for a few days, just to see how the freebies live.
Today?
Your Paid Premier Membership has Expired
Renew
If you choose not to renew at the moment, your library and settings will be retained for six months. If you renew within the next six months, your library and other settings will be what they are now (you would be using the site in free membership mode). If six months pass without you renewing, your library and premier service settings will be deleted.
Do Not Renew
So I go to "Do Not Renew" and from there to the Main page.
As far as I can see, whatever I click from there gets me back to the "Your Paid Premier Membership has Expired" page, even "Sign Out".
The solution turned out to be:
Right-click on something (story, blog entry, any link) and "Open Link in private window". Logging in to the site in the private window fixed the non-private window, permitting all the usual non-premium functions.