I have had similar experiences, but it was the SIN to NRT part. May 5th departure, went though security and when my ticket was scanned, double beeped, sir, we upgraded you to Business Elite, wow, this was a first, looked pretty empty near the gate so thought it was going to be a quiet flight. I made it no more than 5 steps from the counter, when they called me back and said, sir we need to see the pass again. They handed me back my original seat, and said well you got upgraded to Economy Comfort, which I said this was my original seat, what happened. All they could say to me was sorry sir, we do not know.
To boot the NRT to DTW departed an hour and half late since they had some radio mechanical failure, and I got sent an email a few days later asking me to fill out a survey to comment on the late flight due to "Overbooking". The plane was 90% full, give me a break!