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.
Legalitet:
Standard: Ej legal
Pioneer: Legal
Modern: Legal
Legacy: Legal
Vintage: Legal
EDH / Commander: Legal
Pauper: Legal
Explorer: Legal
Historic: Legal
Brawl: Ej legal
Premodern: Ej legal
93/94: Ej legal
Mighty Leap är Legal i dessa block: Battle for Zendikar, Amonkhet