Can you choose a new target for an aura on the stack if the original target gains hexproof before the aura resolves?

When an Aura spell is on the stack, it targets a permanent (or player) as chosen when it was cast. If that target gains hexproof before the Aura resolves, the spell does not automatically retarget — you are not given the opportunity to simply pick a new target on your own.

However, hexproof matters at resolution: under CR 608.2b, the game checks whether the target is still legal when the spell tries to resolve. Since hexproof prevents the permanent from being targeted by spells or abilities controlled by opponents (CR 702.11b), if the Aura is controlled by the target's controller, hexproof does NOT protect against it — the spell resolves normally. If you are an opponent, the target is now illegal and the Aura spell is countered on resolution by game rules (CR 608.2b).

The only way to choose a new target for a spell already on the stack is through an effect that explicitly says you may change a target, such as 'redirect' effects or spells like Deflection (CR 114.6, 115.7d). Simply wanting to retarget is not allowed.

Concrete example: You cast Pacifism targeting your opponent's Grizzly Bears. In response, your opponent casts Ranger's Guile, giving Grizzly Bears hexproof until end of turn. You cannot pick a new target. When Pacifism tries to resolve, Grizzly Bears is now an illegal target (you're the opponent), so Pacifism is countered and goes to your graveyard (CR 608.2b).

If instead you had cast Pacifism on your own Grizzly Bears and it gained hexproof, hexproof only protects from opponents' spells, so the Bears would not be protected from your own Aura and Pacifism resolves normally (CR 702.11b).