You've been assigned a new seat.
So when I gave the gate agent my paper boarding pass the machine beeps and the red light switches on. "One moment sir, you've been assigned a different seat". After checking her terminal and punching some keys, she crosses out my original seat number and writes 6F on the boarding pass.
"How cool!" I thought. As I had reserved a specific seat during booking I knew that row 6 was the las row in Business Class.
While boarding the plane I was excited to check if I was correct; turns out I was! My 6F was indeed in Business Class.
"How cool!" I thought. Doesn't happen a lot to be "involuntary" upgraded.
I was really savouring the moment!
Good thing because just after the plane was pushed back two cabin crew appeared and moved the curtain one row forward.
At least I enjoyed while it lasted.