Does ward trigger if a spell targets a permanent but then has its target changed away from that permanent?

Ward is a triggered ability that triggers when the permanent becomes the target of a spell or ability an opponent controls (CR 702.159a). The key moment is when the permanent first becomes a target — specifically when the spell or ability is cast or activated and targets are chosen, or when an effect causes the permanent to become targeted.

If a spell originally targets a different permanent and then that target is changed to the ward permanent (for example, via Redirect or Willbender), ward does trigger at that point, because the permanent is now becoming a target for the first time under that spell (CR 702.159b).

Conversely, if a spell originally targets the ward permanent and the target is later changed away from it, ward has already triggered (or its window has passed). The permanent is no longer a target, so ward does not trigger again. Ward only cares about the moment a permanent becomes targeted, not about retargeting away from it.

Concrete example: Your opponent casts Lightning Bolt targeting your Creatures with Ward 2. Ward triggers immediately. In response, you use Deflection to redirect the Bolt to a different creature. Ward already triggered when the Bolt originally targeted your creature; the redirect does not create a new ward trigger, and the original ward trigger is still on the stack (though now the Bolt is no longer targeting your creature, making the ward trigger's condition moot if it requires payment to let the spell continue targeting it).

In summary: ward triggers when the permanent becomes a target. Moving a target onto a ward permanent triggers ward; moving a target away from a ward permanent does not trigger ward (CR 702.159a, 702.159b).