The .fdf file in the build describes the layout of the flash device, so that could be updated to generate an 8MB image for the particular SPI flash device on the customer’s board.
The .fdf file specifies the size of the image and how things are laid out, so if you update that file, you can generate an 8MB image. The .fdf file format is described in the EDK II specifications.
However, given that the Crown Bay platform has no firmware descriptor or any other firmware components, as long as the 1MB image is simply flashed to the last 1MB of the device, it should work just fine.
What are you using to flash the image?
If using a DediProg, simply use Batch mode in the DediProg software to just update the last 1MB instead of the whole device.