Hey guys I'm new here and if this isn't the right forum for a question like this feel free to move it.

Anyways a couple months ago I went out to Las Vegas from Newark and while looking at the plane coming home flight history I realized that occasionally it would be a different plane. I figured it was just a one time thing. But now in a couple weeks we are going back out to Vegas and using the same plane back into Newark. And I realize looking at the flights past history that occasionally it's using a different plane.

My question is.. is this normal? For the same flight # to use a different plane occasionally? Or is this a sign that the plane is being repaired? etc?

Here is an added link. The original plane should be a 757-3 But on November 14th a 737-9 was used.
http://flightaware.com/live/flight/COA1579

Thanks,