Hello,
1. Find old topic with description.
2. Run sql query. If your tables has a prefix add it in query.
SELECT *
FROM forums_topics
WHERE tid=X
X is topic ID.
3. Find old description
4. Look at field name.
No need it. I found a column name here https://invisionpower.com/forums/topic/349558-download-pav33-topic-description/
You are right. Description will be removed during the upgrade.
applications/forums/setup/upg_40000/upgrade.php
$description = \IPS\Db::i()->checkForColumn( 'forums_topics', 'description' ) ? "DROP COLUMN description," : '';