Why was the original tagline changed from the movie poster?

I'll go with some reasoning rather than an actual interview from the distributors here. First of all, you already explained the most likely reason for why the new poster was changed at all and why it has a huge Jennifer Aniston superimposed onto the old poster. And that seems also the reason for the changed tagline. The new tagline integrates better with the new poster.

In the old poster you had the leprechaun looking directly at the audience and the tagline thus adressed the audience saying that our luck ran out, because that leprechaun over there will soon kill us. Now in the new poster that break of the fourth wall has been diminished, since we now have a closed scene where the leprechaun basically looks at poor Jennifer instead of us. So clearly her luck ran out and the tagline doesn't need to break the fourth wall either.

The poster gives a more coherent whole this way and doesn't completely look like a bad photoshop of the old poster to smash in the face of a now famous celebrity (even if it practically is just that). By also adapting the tagline, they made that tagline work better with the entirely changed narrative situation resulting from the new poster. Or said differently, you want a big face in your poster? Then better have it make sense to be there. ;-)