User blog comment:MisterWoodhouse/The first migrations to the Unified Community Platform/@comment-3018500-20200812025039/@comment-24006128-20200812131600

The Forum migration script is using a special converter to render the legacy content's wikitext formatting into HTML so that it will show up as is in the vast majority of cases.

Editing the legacy content after migration will break that, but at least we can get it set for historical context.