In order to be eligible to receive the trade-in credit, patrons must be PowerUp Rewards members at the time of the initial purchase, which must take place within 30 days of the game's release date. Then,
No mention of the PlayStation 4 was made in the press release, so whether a similar offer will be made for PS3/PS4 cross-generation games remains to be seen.
Update: Gamestop has informed us that the $50 coupon is delivered in tandem with the purchase of the Xbox 360 version of the game, rather than after the upgrade to the Xbox One version. This article has been altered slightly to reflect that information.