Scripting access to TextDocument.boxText (writing, not just read-only)
In order to convert the currently selected Text Layers into Point Text or Paragraph Text, the user needs to right-click in the Composition panel with the type tool, and choose the "Convert To Paragraph Text" or "Convert To Point Text" submenu.
But if multiple Text Layers of the same type (Point Text or Paragraph Text) are selected the "Convert To Paragraph Text" or "Convert To Point Text" submenu is greyed out.
So it seems the only way to convert multiple text layers is to select each layer one by one and right-click "Convert to..." every time.
If not possible to (or in addition to) enable the "Convert to..." submenu when multiple Text Layers of the same type (Point Text or Paragraph Text) are selected, please give scripts write access to the TextDocument.boxTex property or a new TextDocument.boxTex(bool) method in order to automate workflows.
(Apparently I forgot to link this related suggestion in the original post: https://adobe-video.uservoice.com/forums/911311-after-effects/suggestions/34862911-convert-multiple-text-layers-to-point-text-or-para)