We might talk about data lost or will the data be somewhere else stored?
You uploaded a Word document, edited the Server-Side properties and the result could look like:
The document properties looks as follows:
What happens when you delete the two columns (server-side properties) Year and MyText?
All the data I added to my thousand documents in that document library are lost?
Answer: NO! *
The data will be stored inside the document in Document Properties.
Hint: The Author who works with the document might be responsible for the content as usual but also for the metadata and in this case the server side properties and also the client side Document Properties!
Still be careful because in case you disabled property promotion and demotion, the message box would tell you the true things, permanently deleted.
How to Disable Property Promotion In WSS http://blogs.msdn.com/b/josrod/archive/2009/05/11/how-to-disable-property-promotion-in-wss.aspx
When you upload a docx file to a document library that contains already columns with the same name you have inside your document as Document Properties, you may see converting issues.
See also my second post that explains when you upload a document with document properties that contains the same name you have also a column named. http://blogs.msdn.com/b/joerg_sinemus/archive/2013/09/13/doclib-columns-server-side-properties-and-document-properties.aspx