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.