Sub changeField _
( doc As NotesDocument, oldName As String, _
newName As String )
  If doc.HasItem( oldName ) Then
    Call doc.ReplaceItemValue( newName,  _
    doc.GetItemValue( oldName ) )
    Call doc.RemoveItem( oldName )
    Call doc.Save( False, True )
  End If
End Sub
For example, to change the name of the phone item to officePhone without losing the item's value, a script makes the following call:
Dim doc as NotesDocument
'...set value of doc...
Call changeField( doc, "phone", "officePhone" )