The Yankees will take on the Orioles tomorrow to complete a suspended game from June 28. The game will resume with the Yankees leading the Orioles 8-6 in the top of the eighth.
If Alex Rodriguez hits a home run in the balance of that game (he'll be on deck when the game begins), it will become home run #493. Which means the home run Rodriguez hit back on Wednesday in Kansas City would no longer be home run #499, but rather home run #500.
That's because baseball's rules require that no stats from a suspended game be counted until the game is over and all events that take place during the suspended game are considered to have occurred on the day of the original game, i.e. June 28.
Orioles manager Dave Trembley won't say who will be coming out to start the suspended game. The pitcher in the game at the time, Chris Ray, has gone on the disabled list. But Miguel Tejada has gone on the DL and come back in the interim.