Are you using the Mail Merge helper from Word's Task Pane?  In Step 3 of 6,
you have the option to select from Outlook contacts.  If you choose this
option, you will be asked which contacts folder you want to use, and then
every time you reuse the document you will be asked again if you still want
to use that contacts folder.  This way you will always have a current list of
your contacts to choose from for the merge.