Wait isn't the ticket under Airport Control 24 hours prior to the flight and releases the lock from the third party vendors? So if BA changed a portion of the ticket wouldn't Expedia be out of the picture as now the airline touched the reservation and it becomes their property? This is for cancellations that occur 24 hours and up to flight time. The OP's flight cancellation took place a few weeks before so the following below must apply.

I thought the OTAs only work for the weeks days and months leading up to the reservation but once Online Checkin becomes live the OTAs have no control as how else would the Online Checkin be able to process the passengers and add them to the manifest?

Now I always do my best to book directly with the airline to cut out the middlemen. If I were the OP I would ask BA to work with Expedia to get full ownership of the reservation. Remember Expedia deals with other alliances unlike BA they could really only book you on AA, BA and other One World Partners and in the very rare case endorse a ticket to Delta.

WIth IRROPs that occur prior to the travel date Expedia must rebook the passenger using the authorized flights suggested by BA or refund the full cost of the flight. I once had my international itinerary changed and was booked under a third party and I had the airline take ownership of the itinerary after the third party OTA refused to adhere to the airline's rebooking policy. So if Expedia refuses to do what is obligated of them I would contact Christopher Elliott an Ombudsman www.elliott.org.

What Christoper Elliott suggests is to call Expedia again if they fail to fix the ticket then you work up the chain using his Executive Contacts to get them to do so and contact BA to inform them if Expeida is not doing what is obligated of them. Again I would suggest int he future just to book directly with BA even if its a bit more $$$
The OP should try calling the Rebooking line and let them handle it even if it means they initiate a three way call with Expedia.

travel date is sept 19th, op is way before the ticket is under ba control

I pine for the OP. His issue is utterly simple to solve. His ticket can be rebooked, as some have already said, in the lowest available booking class in the appropriate cabin and the exchange ticket or revalidation all completed in approximately 3 minutes.
However, when you book a discounted ticket (eg through an OTA) you simply must expect a discounted service too, exactly as we are seeing here, unless you are very, very lucky. These OTA's often only earn a few pounds on large value tickets and are simply too scared to re-issue a ticket due to a lack of training or fear of an ADM (agency debit memo) landing on their desk for several hundred pounds for making an error. All this despite very clear rebooking guidelines supplied to them by BA.

Yes I know that but I was explaining how the Ownership transfers from the OTA. The OP is obligated to work with Expedia unless they flat out refuse to help then the OP needs to work on a solution that is good for him/her.

My guess is that BA notified Expedia of the cancelation at least 14 days before 19/9 and Expedia failed to relay.
Expedia is the only way to to get rebooked. You have to insist with Expedia.
But the remaining two BA flights on that day are fully booked in PE. Hence, it will be a big hassle.

I am not sure which 'window' your travel agent is talking about as you can get rebooked pretty much immediately. There is a way to get to SDF on 07/12 (MAN-PHL-SDF), and there are seats available. You can also opt to go to LEX (via LHR and ORD). Yes, you will keep business class seats even if there is no reward availability on new flights.