How to convert doc to docx

1.way (Microsoft Office NOT required)

download 
-Microsoft Office Compatibility Pack - here
-Microsoft Office Migration Planning Manager - here, (older version here)

Install both programs.

edit ofc.ini from the second program. It is in the folder where you installed it.

set 
FullUpgradeOnOpen=1
fldr=d:\source_dir_with_doc_files
DestinationPathTemplate=d:\destination_folder


start ofc.exe

source:
http://blogs.msdn.com/b/ericwhite/archive/2008/09/19/bulk-convert-doc-to-docx.aspx

or use some of these programs:
-doc2x (command line tool) here
-libre office here
-wps office here (The one from wps.com has expire date for saving as doc/docx.)
-abiword here (Can be used from command line)