When you book a flight as an elite member, you are put on the "requested" list for a first class upgrade. When you checki-in for the flight, this "request" goes to the "standy-by" mode and no seat is shown since you are "standing by" for that first class seat. You should still see an additional line, showing your original coach seat assignment, which says "confirmed". Thus the same flight, AFTER online check-in, should have two lines for the same flight segment - one which is "confirmed" in coach" with a seat assignemnt and one which is "standby" in first class with no seat assignment.

Your original seat on this particular flight may have disappeared because something else may have happened in this particular reservation- a plane change. Thus, you may still have a seat but it may not be showing.

If I were you, I would call SMS and ask what happened to the original coach seat assignment is on the flight and make sure that you have one so that you are not in the dreaded "midle seat"..... unless you personally requested it!

Good luck!!