From version 3.7.0 Secure Fields provide a method to automatically migrate secure fields to regular custom fields. You have to be a JIRA administrator to perform this operation.
Back up your JIRA instance, as the migration process is irreversible.
Log in as a user with the JIRA Administrators global permission.
Navigate to Admin panel → Manage apps → Secure Fields → Unsecure Secure Fields.
Select fields you want to migrate.
[optional] Check the Migrate Secure History to field history box.
When you are ready click Migrate button.
Restart your JIRA instance.
Run JIRA reindex.
Secure Fields will be migrated to their corresponding Jira standard fields.
Secure List type can be migrated to Radio Buttons or Select List (single choice) field.
Secure Multi Select List can be migrated to Checkboxes or Select List (multiple choices) field.
Most Secure Field types can be unsecured at the moment. The exceptions are Secure Version Picker (single&multiple version), Secure Component, Masked Secure Text and Secure Password.