This can be enabled on a per event basis. The confirmation email that is sent after a successful registration will include a link to allow the registrant to go back and make changes to their registration, provided the resulting total amount is equal to or higher than their original total.
For example, instead of purchasing just one add-on, if the registrant decided they would like 2, this is possible. The link will bring up the original form so that changes can be made, and then the final payment step will request payment of the difference. If the registrants decides to cancel an add-on or downgrade to a lower package, they cannot do so online and will be prompted to contact the event chair as this will require processing a refund according to the event's policies.
For more information on MyEventRunner or for assistance, please contact our Client Services Team.