Psych Up copies both the positive and
negative changes to the target's stats
and replicates them onto the user. The
target's Attack, Defense, Special
Attack, Special Defense, Speed,
Accuracy and Evasion are copied. Any
changes made to the target's stats
after Psych Up is used will not affect
the user's stats.
So no you will keep your stat changes that you gained after using psych up. So Celebi would get 3 Sp.atk boosts and 1 Sp.def boost and kill cofagrigus.