Officially I believe it means no changes whatsoever, although it may depend on your status with the relevant carrier as to whether you could get away with a time change. But note that this only applies to tickets issued for travel originating in TC1 (the Americas). Tickets issued for origination anywhere else have no restriction on changes before departure (though a ticket reissue is required - BA would charge GBP25 for this; others would do it FOC). There is also the restriction that the ticketing cannot change within 7 days of departure for economy class bookings, but that doesn't apply to your DONE5.
Correct for before travel commences. Afterwards it is a per change, not per segment change. In other words you can completely redo the whole ticket from the current point for a one-time US$75 (unless you ask BA in which case they will charge you US$75 plus an additional GBP25 as they do for all ticketing actions). I have completely turned round an AONE3 after the first flight, changing the order of the continents and all of the routing in this way.