It is not reduced when a player moves up a level but rather when he earns money under the Bonus Incentive Plan. The bonus incentive would be for milb advancement EACH beginning season, not during season. I was confused as son moved up and it wasn't taken out, though he earned more, but that was during season.
Thanks, I learned something today!
Here's the MLB plan.
http://mlb.mlb.com/mlb/officia....jsp?content=guide#1