This is not eligible for SDC because it is an international itinerary (other than JFK-LHR). Informally, AA will allow SDC after all international segments have been flown, but that is not the case here. Thus, what OP wants is a waiver of the change fee.

If OP misconnects, AA will need to rebook and that will depend on what is available at the time. In particular, likely to be lousy seating because the good seats will have been assigned.

I would do as aztimm suggests, with the exception of, if denied a fee-free change, asking what the new ticket costs with an earlier flight. It is possible that the earlier connection is cheaper and that will set off against the change fee, so worth determining whether that makes the cost tolerable.