When Enthralling Victor enters the battlefield, gain control of target creature an opponent controls with power 2 or less until end of turn. Untap that creature. It gains haste until end of turn. (It can attack and this turn.)
If you stare at the sun for too long, you'll lose your ability to see clearly.
Once Enthralling Victor's ability resolves and you gain control of a creature, raising its power above 2 won't cause you to lose control of it. Similarly, it doesn't matter if Enthralling Victor leaves the battlefield or you lose control of Enthralling Victor. You'll keep control of the creature that was the target of the ability until the end of the turn.
Enthralling Victor's ability can target a creature that's already untapped. You'll still gain control of it and it will gain haste.