What happens when a guided tau unit is forced to select new targets?

[Blight1]

So can't seem to find this anywhere but what happens when a tau unit selects its targets, gets guided against said targets, and then has those targets become ineligible because of a strat.  From what I can see they still obviously get to pick new targets but they've already been guided for the unit they can no longer shoot at and thus would suffer a penalty to hit against their new target.

[Wormwoods]

That's my basic understanding, yes.