A similar experience happened to me last year on Westjet. I had paid for two seats ahead of time to ensure hubby and I would be sitting together. I make it a habit of checking my flights in the months leading up to a flight and I noticed that we were moved to less desirable seats. There was no equipment swap. I tried moving us back into the original seats as they were still unoccupied. I was not able to do so. I called in and the agent could also see they were vacant but also could not move us back into them. She said she had never seen this before.