Codeshares are irrelevant for change fees which are always based on the ticket. So yes, you can change these w/o fee but with an applicable fare difference (if any) irrespective of codeshares. A3 flights or any other flights even with non-*A have no influence on that. You can make changes any time, but changing mid itinerary can become complex depending on circumstances - you still need to abide to the original ticket rules. If you change prior to departure then essentially you are buying a new ticket and applying the value of the old ticket.