You are completely certain about all of the following?
- You wanted to reroll Darkrai.
- Darkrai was in your party, and Regice was in your box.
- You wanted to lock Special Attack, but chose to lock Special Defense instead, but neglected to mention this in your original post.
- While you were interacting with the NPC, Darkrai was in your party.
- The reroll preview showed you Darkrai.
- Regice was suddenly moved to your party after the reroll preview, and Darkrai was sent to your box.
- Regice's stats were changed.
Remember, false bug reports constitute a bannable offense. Requesting to have your Regice restored to its previous state does not.