Giving a creature flying after it’s already been blocked won’t change or undo that block. If you want to affect what can block the creature, you must cast Mighty Leap during the declare attackers step at the latest.
Lovlighet:
Standard: Ikke lovlig
Pioneer: Lovlig
Modern: Lovlig
Legacy: Lovlig
Vintage: Lovlig
EDH / Commander: Lovlig
Pauper: Lovlig
Explorer: Lovlig
Historic: Lovlig
Brawl: Ikke lovlig
Premodern: Ikke lovlig
93/94: Ikke lovlig
Mighty Leap er Lovlig i disse blockene: Battle for Zendikar, Amonkhet