Retaining .doc files rather than .docx files in a SPO Library

Hi Kathy Grube ,

I am not seeing .doc files being converted, I have had the opposite issue where .doc files migrated to SharePoint don't work in co-authoring etc and have had to convert them to .docx. 

If you are having an issue the user can open the Word App and use the Save As to save as a .doc file when they need to send to the website.

Kathy Grube 

DOCX

DOCX is part of Microsoft Office Open XML specification (also known as OOXML or OpenXML) and was introduced with Office 2007. DOCX is a zipped, XML-based file format. Microsoft Word 2007 and later use DOCX as the default file format when creating a new document. Support for loading and saving legacy DOC files is also included.

DOC

DOC is the default format used with Office 97-2003. DOC is a Microsoft proprietary Binary Interchange File Format. Microsoft Word 97-2003 uses DOC as the default file format when creating a new document. Support for loading and saving DOCX files can be added using an add-in.

https://support.office.com/en-us/article/file-types-supported-for-previewing-files-in-onedrive-sharepoint-and-teams-e054cd0f-8ef2-4ccb-937e-26e37419c5e4

https://www.techmikael.com/2014/01/keeping-versions-when-upgrading.html

I did a practical analysis on this issue, where I copied a doc file to SharePoint library & it still supports to download 
Copied below file to SPO Socument Library

Correct me if I'm wrong in understanding