Some retailers offered cross-gen promotions near the start of this generation to ease the transition, and it seems GameStop is still planning to do that when releases are staggered. With the PlayStation 3 and Vita versions of MLB 14: The Show out now and PS4 not due for another two months, it will offer a trade-up price for the newer version.
According to the announcement, upgrading your PlayStation 3 version for the PlayStation 4 version next month will grant you enough credit to purchase it for $19.99. Pre-ordering also gets you $20 of in-game currency ("Stubs"), so if you planned on purchasing any perks that will serve as a boost. It's not a bad deal if you were holding out for the more powerful version but want to get baseball season started now.
The special price for trading in the last-gen version makes it a similar (but more pricey) version of the promotion that ran last year with games like Call of Duty: Ghosts and Assassin's Creed 4. You'll need to be a PowerUp Rewards member to take advantage of the deal, however.