Just did this at LAS yesterday. Took 2 minutes. However, it took about 30 minutes for the guy to figure out how to put me on an earlier flight (I am Gold). First he wanted to charge me a $75 change fee, then he wanted to charge me a fare difference (which isnt a rule on same-day changes), and finally he had to call WS help desk to get it all sorted out. He asked to see my Gold card, and some sort of proof that same day changes were free. WS staff should know their top tier better.