I do it all the time. Never a problem, as long as (1) you are there before the cutoff time for the earlier flight, and (2) the earlier flight isn't sold out.

I usually go through security first and then straight to the customer service desk, just to be sure I don't infringe on (1) above. Neither of these are typically a problem, because I've been Gold all year and now Platinum, so I can get the the head of the line in both security and customer service, but if you're not then you need to account for the waiting time at both of those steps.

As noted above, as long as you meet those 2 criteria, the customer service agent will just happily cancel your reservation on the later flight and issue you a new printed boarding card for the earlier flight (after you pay the fee of course if you don't have a Flex fare or aren't Gold or Platinum).

Cheers
TRJB