RE: Pokémon X / Y : Playthrough Questions & Answers
As far as I know, you could keep the same EVs that you had at L70 when you are reduced to 50. The game doesn't go back in time to see what your EVs were at that point.
Jakeremix said:Let's say my Greninja is level 70, but it isn't properly EV trained at all. If I used a Reset Bag and trained him how I wanted him trained, would it make any difference when I'm battling with a friend or on random matchup and my pokemon are lowered to level 50? If not, does that mean all my pokemon must be EV trained before level 50?
As far as I know, you could keep the same EVs that you had at L70 when you are reduced to 50. The game doesn't go back in time to see what your EVs were at that point.