I have to ask what you are trying to do. You can force the document to have the odt extension by doing Save As and turning off the Automatic File extension. You can then name it XYZ.odt. The file will then appear with the Writer icon. But that will not do anything to the format of the file and when you open it, it will open in Impress. If you explain your goal, maybe we can help you get there.
AOO 3.4 or 4.1 on MS Windows XP ( before 2013-08-03) or Windows 7
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