You may cast Windshaper Planetar outside of a declare attackers step. If Windshaper Planetar enters the battlefield outside of a declare attackers step, its ability simply doesn't trigger.

Reselecting which player or planeswalker a creature is attacking ignores all requirements, restrictions, and costs associated with attacking.

Reselecting which player or planeswalker a creature is attacking doesn't cause “whenever this creature attacks” abilities to trigger.

If you reselect which player or planeswalker an attacking creature is attacking, that creature is still considered to have attacked the player or planeswalker as declared, but it is now attacking the new player or planeswalker.

If an ability targets something controlled by the “defending player” of an attacking creature and the defending player for that creature changes before that ability resolves, the ability won't resolve because its target has become illegal.