There are any number of reasons for the downgrade. The obvious one is a FAM claiming the seat. In that case, it has nothing to do with the price paid for the seat but rather the seat chosen. If it was due to a FAM, I’m guessing the OP was in an aisle seat. The second thing that came to mind was somebody in a protected class took priority. The protected class could be somebody who was bumped from an earlier flight for whatever reason. Not knowing the exact rules AS employs in this case, I couldn’t say how OP ended up winning that lottery. But congratulations, you’re a two time winner.