I haven't seen it really advertised anywhere, but on the MLB.com BallPark App they actually have a section where you can upgrade your ticket for a fee. I haven't looked into it with much detail, but it looks like you can do it for any game with the upgrading opportunity starting a couple hours before start time. The cost to upgrade seems to depend on game, area you are upgrading to, and availability in that section. Today for the field level it was $40 to get into the diamond box(edge of infield), $23 for field infield, and the cheapest was the Club Infield coming in at $7. It is an interesting concept and may save some cash if you buy some bargain terrace tickets off StubHub or get the cheap Friday bleacher seats as your primary ticket.