I did see the same thing, Adam Dunn increased all his attributes by 8 in the first season I simmed. I'm not sure if there is a solution. It seems that most overalls were reduced during the re-rate process. As a result, the game is interpreting a 60-something Overall with B potential, and moving him up closer to his potential rating, despite being on the wrong side of 30.
A possible solution could be reducing the potentials of the old, over-the-hill guys, by my understanding is that future draft classes are based on the players who retire.