The below query should do the one time migration for you. CB 1.x didn't store the signature and just sent it directly to Kunena. In this case CB stores the signature then sends it to Kunena. This avoids the signature being lost as well as gives us the ability to support other forums (important for the future) since we store it our selves.
We probably should've accounted for this in the migration though and have added a bug ticket to fix that part of the migration.
forge.joomlapolis.com/issues/5441
Code:
UPDATE `jos_comprofiler` AS c SET c.`FIELD_NAME` = ( SELECT k.`signature` FROM `jos_kunena_users` AS k WHERE k.`userid` = c.`id` ) WHERE ( c.`FIELD_NAME` IS NULL OR c.`FIELD_NAME` = '' )
Replace jos_ with your table prefix and replace FIELD_NAME with the name of your signature field created in CB. This will migrate the Kunena signatures to CB if they don't have one stored in CB. This should be ran directly on your database (e.g. using phpmyadmin).
Please be sure to make a backup of your database before running this.