Yes, you may change your race distance online up to 10 days prior to the race. All other requests must be made in person at the expo.
There is no processing fee to change races. For downgrades, no refunds will be issued. For upgrades, the additional registration fee will be charged.
Per USATF rules, if you decide to change races unofficially, you will be disqualified. You must run the race for which you registered.