The current conversion script reads

-- SAK-13584 Further Improve the Performance of the Email Archive and Message API. Note you have to run a bash conversion script on your old mail archive data for it to
-- appear in the new mail archive. The script is in the source as mailarchive-runconversion.sh. Please see the SAK for more information on this script or SAK-16554 for
-- updates to this script.

CREATE INDEX IE_MAILARC_SUBJECT ON MAILARCHIVE_MESSAGE
(
       SUBJECT ASC
);
ALTER TABLE MAILARCHIVE_MESSAGE ADD COLUMN (
       SUBJECT VARCHAR (255) NULL,
       BODY LONGTEXT NULL
);

This fails when I apply it, because the column subject does not exist and therefore cannot be indexed.

I had to flip the order to

ALTER TABLE MAILARCHIVE_MESSAGE ADD COLUMN (
       SUBJECT VARCHAR (255) NULL,
       BODY LONGTEXT NULL
);
CREATE INDEX IE_MAILARC_SUBJECT ON MAILARCHIVE_MESSAGE
(
       SUBJECT ASC
);