Can attendees access their registration and make changes after they've submitted a completed registration?
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1There may be some useful information in my related question here: civicrm.stackexchange.com/questions/9065/… – Eli Lisseck May 4 '16 at 14:41
This is not out-of-the box functionality. Have a look at these threads:
Recommended process for credit card additions/'edits' to event registrations?