How DOC to DOCX Conversion Works
The converter opens the binary .doc file, reads its text, styles, tables and images, and writes them into a .docx package. That package is the format Word has used since 2007.
Nothing about the content changes on the way. The upgrade is about the container: DOCX is a zip of XML parts instead of one opaque binary blob.
Why Upgrade From DOC
Word opens .doc files in compatibility mode, which switches off newer features and sometimes changes how the document is laid out. Converting removes that banner and unlocks the full editor.
DOCX files are also smaller, survive corruption better and can be read by scripts and web apps, because the text sits in XML rather than a proprietary binary structure.
Common Use Cases
Clearing out an archive of documents from the 2000s so they behave normally in a current Word, Google Docs or Pages.
Preparing a file for a system that accepts only DOCX, such as a modern applicant tracker, a content pipeline or a document API.
What Carries Over
Text, headings, styles, lists, tables, images, headers, footers, footnotes and comments carry over. Track changes and bookmarks survive as well.
Very old features have no equivalent in DOCX: WordArt from the 1990s, some form fields and macros in a .doc file do not come across. A document that relies on macros needs the .doc kept as a backup.
Tips Before You Convert
Keep the original .doc until you have opened the result and checked the pages that matter. Conversion never edits your upload, so the original stays as it was.
If the document contains a mail merge or a form, open the converted file and test that part first, since those are the pieces most likely to need reconnecting.