I checked the local Ford parts department and several online Ford OEM parts suppliers, the lowest price I could find for the traction batteries for the 2017 FFE is about $19k. That price is more than what I just paid for my certified 2017 FFE. So if you really want the 33.5 kWh in your older model FFE you are better off buying a used 2017 or finding a wrecked 2017 and stealing the battery out of it.
Another important bit of news is that it looks like the 24 kWh battery has been COMPLETELY DISCONTINUED (without Ford telling anyone!). I checked several model years in addition to 2017 and all model years had the same traction battery part numbers and prices. I checked the prices on the 24 kWh battery pack just last year and it was down to $8k so I know Ford now has to be selling the larger battery packs exclusively.
So for all of those that are interested in putting a 2017 model battery in your older model, it doesn't look like you have a choice, if you buy a new battery for your FFE it will have to be a 33.5 kWh battery pack. I haven't heard of any discounts if you want to replace your battery pack with the larger one and I don't expect to see one. It looks like for now Ford is more interested in trying to get you to buy a 2017 model than selling batteries for older models.