My earlier post was wrong. Here's the info from easyjet.com:
If you arrive early at the airport for your flight home, you may transfer for free onto an earlier flight home on the same day as the flight originally booked, provided this flight has seats available and you transferring won’t cause a delay.
Please make your way to the easyJet sales desk to request a free transfer to an earlier flight. The only stipulation is that you must have already made an outbound journey with easyJet and be returning to your original departure airport. For additional convenience if your original point of departure is a London airport, where available we will allow you to return to any London airport (i.e. Gatwick, Luton, or Stansted), which gives you extra flexibility if you’re travelling home on public transport!
I'll amend the earlier post.