Presuming you know the document password, have you tried removing it from the document?
http://www.ehow.com/how_6776384_remove-word-password.html
Here is a link describing visually how to remove a known password from a Word 2003 document password (ignore all the links on the page, as they are selling software to remove passwords which you obviously don't need):
http://www.daossoft.com/knowledge/how-to-clear-microsoft-word-2003-password.html
My suggestion is to totally unprotect a copy of the document and then complete your edits. Once you are done editing, you should be able to re-apply the password to the copy and re-save it if you want to.