Sometimes a talk subpage contains actual discussion of its related article. so it should become a Flow board. Other times it just holds related information for its parent discussion page, so it should remain a regular wikitext page.
Example: the conversion of a namespace to Flow on OfficeWiki did not touch the subpage https://office.wikimedia.org/wiki/OfficeWiki_talk:WikiProject_OfficeWiki/guidingprinciples