Error parsing DDL for schema.table

[N BD]

I have production dbs using Mysql 5.5.31-0ubuntu0.12.04-1 (I know its old, will be upgraded soon).

I used a too advanced version of mysql workbench which allowed me to add a comment to key. (Now reverted to WB 6.3 which should be compatible with mysql 5.5)

Now when I 'Alter table' on one of the tables I get Error parsing DDL. When I click it shows the error is in the comment of the key (see below)

KEY `for cron (product ready)` (`a`,`b`),
KEY `subproducts` (`a`,`b`,`c`) COMMENT 'find products by the subproducts joiner'
) ENGINE=InnoDB DEFAULT CHARSET=utf8

The question is how can I remove the comment of the key without mysql rebuilding the table and without access to the mysql workbench alter table UI. This is an important production server and I would like to avoid alter table which will scan all the table.


Thanks