I wonder why they did not run the curl command when the upgrade to 3.2 is performed rather than hoping someone would see web page. It negates automated updating. Lesson is to remove graylog from yum-cron.
it looks simple from a user perspective but is it not easy to solve. That step is only needed if you want to make use of the gl2_accounted_message_size field. If that is not important for you - leave it and on the next rotate of indices that field will be added with the correct mapping.
The system will not break if you did not do this. In addition - only bug-fix releases are able to be updated automatically. Dot releases should always include that you check the upgrade notes and if configuration has changed or not.