CiviCRM Community Forums (archive)

If the multi-select  custom field "belongs" to Contact records ("Used For = Contacts"), then it previously selected value(s) should show up on subsequent event registration forms along with their other saved data. (If the custom field "belongs" to Participant records - then it will not show up - as each event registration creates a new participant record for the contact.)

Please see if you can recreate the issue of the multi-select value not showing up on the 1.9 alpha sandbox - and file an issue if it recreates.
http://sandbox.civicrm.org/drupal/

You can not change the Data and Input types for an existing custom field as this could damage the integrity of existing "saved" data. Specifically for multi-select fields, the "value" data is stored as a delimited list.

Check the "Used For" value of the Custom Data Group that your field is part of to verify whether you are extending Contact records or Participant records with any given custom field. You can see this from Administer CiviCRM >> Custom Data (for example, on demo...http://demo.civicrm.org/drupal/civicrm/admin/custom/group?reset=1).

Each time someone registers for an Event, a new participant record is created. You can test this by doing it yourself and then viewing your contact record and clicking the Events tab.

We recognize that this behavior (always inserting a new participant registration) is not ideal for some situations - but we haven't tackled the complexity of registration "updates" - which is really a different type of action. Updating ones registration may or may not involve a financial transaction for example.