I can't undo my changes after I save my document in a previous file format

Symptoms

After you save your document in Word 97-2003 format, the Undo button appears dimmed, and pressing CTRL+Z doesn't undo your changes.

Cause

When you save your document in Word 97-2003 format, you can't automatically undo any changes that you made before you saved the document.

Workaround

Remember – the original file is still there after you use the Save As command to save in the Word 97-2003 format. Although you won't have any of the work that you've done since you saved the file in the earlier file format, you may be able to salvage some content from the original Word 2007 document.