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When do you choose to add stratagems out of your sideboard? When declaring characters or when sideboarding battlecards?
Stratagems are played concurrently with the starting team for a game. This is required otherwise cards like Master Converter and Aerial Enhancements would not effect the starting team, leading Windsweeper to be sad when Blitzwing and Tracks don't count as Planes.
Source: Aequitas Rules Roundup 2021-01-03
For the purposes of Windsweeper's alt-mode ability, is Blitzwing a Plane on your starting team?
No. Your starting team are the visible cards you present to your opponent at the beginning of the game to show you have an acceptable numbers of stars. These are the sides of the cards that show their star cost.
Given this, Blitzwing is a Tank on your starting team unless you are playing the Master Converter stratagem, in which case Blitzwing is both a Tank and Plane on your starting team.
Source: Aequitas Rules Roundup 2020-09-14