If it's before their stated deadline to submit proof of time, you can update it. I believe most recently this has been done via Active.
I just checked, and I received an email on Nov 11 about submitting/updating proof of time for the Dark Side Half by the deadline. The email had a link in it to Active where the change could be made...
Proof of Time
All challenge and half marathon participants who plan to finish the race in a time less than 3:15 and desire start corral placement must submit a valid proof of time by January 19, 2016.
(No further action is required if you have already submitted.)
NEW: To submit a new or revised proof of time, you must logon to your race registration and update your profile. Click the "Submit Now" Button to logon.