Word 2007 Mail Merge – How to Change Read-only Data Source

I am using Word 2007/Windows XP and am having an issue with Data Sources. The main and data source documents were created in Word 2002 and are on a network share. When I open a Data Source document in 2007, the file is opened as read-only and I cannot save my changes. I did not have this problem in Word 2002. Any assistance would be appreciated. Thank-you very much.