Yes, it is absolutely possible to make changes after checkin. The catch is, you can't just do this self-service, once you've been checked in you will almost certainly need assistance from airline staff to make any changes.
Since the plural of anecdote is data, a couple of cases where I've had my flights changed after checkin, from pedestrian to more unusual:
- Flying in from NY, I missed a connection at SFO, so I was given a new boarding pass at the lounge for a later flight. Very common.
- Flight to Jakarta with a long-ish connection in Bali, both boarding passes issued at origin. I arrived well ahead of schedule and realized I could catch an earlier flight, so I talked to the lounge staff and they made it happen. Not so common but still easy-peasy.
- Checked in online in the morning for a flight at night, but a family member suddenly gets sick. I called up the airline, they "un-check-in" us, split the itinerary so the rest could still travel, and issued new tickets. Took a while, but straight forward.
Finally, the story you mention that made the rounds in 2014 involved somebody supposedly using the China Eastern first-class lounge in Xi'an over 300 times in one year. Sadly if unsurprisingly, it has been debunked, China has strict security at airports and the article claims that no changes are allowed once your physical boarding pass is issued. As noted above, this is actually false, but what is true is that they would not have been able to quietly change their flights by themselves, and they certainly would have been noticed if they had asked staff to make changes 300 times in a row!
