Can attendee data be updated after registration?

Yes, attendee data can be updated after registration. A registration can be created or updated, and Signal can watch for Registration Create Update to send those changes onward. Signal Integration with Eloqua Guide

When a registration is updated, Certain can process changes to registration data and pass them to connected systems through Signal. The registration record also supports editing after creation in the registrations area. General FAQ - Managing Registrations Signal Integration with Eloqua Guide