It is a bit odd that they shoehorned in an existing commercial board rather than just put the FPGA on the main board or even designed a custom daughter card if upgradability was a concern. As it is, they are now tied to the availability of boards with the same board-to-board connector and a similar form factor.
They used a commercial board that implements the Vita 57 bus, which is the standard way to interface with an FPGA [1]. So if they need replacements down the road, they just have to get one that implements the same standard, and standards for industrial equipment tend to stick around for a considerable amount of time.
The Parallella computer, which looks a lot like a Raspberry Pi, has a Xilinx Zynq chip similar to the one in the BBC's new codec. So it's almost true.