For flying to work as an evasion ability, an attacking Goblin Balloon Brigadeâ€™s ability must be activated before the declare blockers step begins. Once Goblin Balloon Brigade has become blocked, giving it flying wonâ€™t change that.
You can activate the ability repeatedly during a turn. This generally has no additional effect, unless some other effect (like that from Adarkar Windform has caused it to lose flying since the last time the ability resolved.
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