Excel overwrite problem

I have a school network with several office users. Just recently the users cannot overwrite an excel file or a word file on a network drive. They can create a new file and save it OK. The files were saved as xls or doc, probably from Office 2003. When the files are opened and modified, they cannot be saved with the same name (overwritten). The users are using Office 2007 or Office 2010. Each time they try a tmp file is created. I understand that office saves the overwrite file as tmp then renames it to the original name when it completes. The users can save the files OK if they change the extension to xlsx or docx. They can then delete to old xls or doc files. Why does it not work like it did before?

I have googled and found others have the same problem. Of course, the first question is ... what has changed on the network? Several things have changed in the last week, but others with the same problem did not have these changes. I originally had 3 DCs; one 2008 and two 2008 R2. I ran dcpromo on the 2008 dc and removed it from the network. DHCP is on one of the 2008 R2 DCs. The removed DC had been running WINS. I changed the options in the DHCP scope to switch WINS to one of the remaining DCs.

The files are on a mapped drive on a 2003 server. I plan to move the files to another shared drive on a 2012 server, but have not done that yet. Of course, I am making the updates and changes during the summer break.

Although the users have a workaround, I would like to know what can be done to fix the problem. Any help would be appreciated. Thanks in advance.