After you set up the legacy form fields and other fields in the document, you must protect the document for forms. Then it operates as you expected.
To protect the document, you can click the Protect Document button (in Word 2007) or Restrict Editing button (in Word 2010) on the Developer tab to open the Restrict Editing and Formatting pane; check the box for "Allow only this type of editing";
and choose "Filling in forms" in the dropdown.
A quicker way is to go to the Customize Quick Access Toolbar dialog and add the Lock command (from the All Commands category) to the toolbar; that button toggles the protection on and off.
Jay Freedman
MS Word MVP FAQ:
http://word.mvps.org