Upgraded @ gate from Flex Y to PY on SCL - YYZ a couple weeks ago.
Took my new boarding pass (14D, aisle), boarded. Sat down and was settling in and seated for 10 minutes or so.
Another pax boarded and I could see / hear him making a bit of scene with the agent at the door. He was travelling with his wife and wanted her a guaranteed aisle seat in PY as he was upgraded to J. They told him to go back up to the gate and speak with agents.
5 minutes later an agent comes down and hands me a new boarding pass moving me to 14E, a middle seat. The gentleman in question then took 14D and said his wife was in J. So I suspect they re-processed the upgrades and upgraded his wife, placing him back in PY, and myself in her seat.

Fairly frustrating as obviously would have preferred the aisle for the 10 hr flight...
Anyone had a similar experience being displaced after boarding / receiving an upgrade?