Now that you mention that, I realize that I should have guessed what was going on, having been caught in a similar situation several years ago. My plane's old Y configuration had 2 seats at the sides, and the new one had 3 seats at the sides. When, without advance notice, my plane turned out to be the new version on the day of the flight, my B aisle seat had turned into a B center seat. Quite a few grumpy pax were berating the poor FAs, and not all could be relocated. I very politely asked to be moved if possible, waited patiently, and was rewarded with an aisle seat and a free glass of "champagne."