Hi Leah,
Thanks for reaching out!
I did some testing on my end but was not able to recreate the issue. As long as the user is logged into the account they used to RSVP with initially, and they change the reservation by going into the initial reservation (for example, by clicking the “View your RSVPs” link on the event) the new response should override the previous response. If you haven’t already, would you be able to do a quick test using this method to verify that the response is not being overridden?
If a new response is being created using the method described above, this could be due to a theme/plugin conflict. Could you try switching to a default WordPress theme such as Twenty Seventeen, deactivating all other plugins besides ours (at least The Events Calendar and Event Tickets), and trying again?
Let me know what you find out!
Thanks,
Jennifer