(1) You can confirm the flight now -- it's just a rebooking at the current prevailing rate. That's your "change fee".

(2) If you're A-list or A+, you can confirm it for free day of (another thread going on around now re: when to call to do that).

(3) Or you can confirm at the airport -- A/A+ it's free. If no tier status, then no guarantees. If the flight is looking pretty open the counter agents have a bit of discretion -- but some may just insist you pay the fare difference (i.e. see #1 above).