If the opponent's monster is no longer on the field (if it was destroyed by battle, or if it was a token), use its original DEF.
Compute the Life Points you gain based on the attacked monster's DEF at the end of the Damage Step. If the attacked monster is no longer on the field, refer to its original DEF.
Even if "Hyper Psychic Blaster" is removed from the field before the end of the Damage Step (example: by a Flip Effect), its effect to gain Life Points still activates.