Technically he can, the commissioner can make any changes at his will that he deems to be vital to the survival of the league. This is why he gets so much flak over handling the steriods issue. He could have implemented a program at any time then forced the MLBPA to publicly denounce steroid testing, which would have been a disaster for them, or accept it. Logistically, it would be a foolish move. I read somewhere that Louisville Slugger, or whoever the main supplier of bats is, along with other bat manufacturers, wouldn't immediately be able to keep up with the demand for ash bats if a change were made without notice. In fact, I think they said they'd need 12 months lead time to change up their production and procurement processes to adjust to the shifted demand. I don't know if I buy 12 months. I bet they could get their act together in an offseason if MLB went ahead without then and started soliciting new companies to make bats. Regardless, if he pulled the trigger on such a move without notice, they would run out of bats fairly quickly.