To work as an evasion ability, an attacking creature must already have flying when the declare blockers step begins. Once a creature has become blocked, giving it flying won’t change that.
Legalitet:
Standard: Ej legal
Pioneer: Ej legal
Modern: Legal
Legacy: Legal
Vintage: Legal
EDH / Commander: Legal
Pauper: Legal
Explorer: Ej legal
Historic: Ej legal
Brawl: Ej legal
Premodern: Legal
93/94: Legal