FPGA can only be reprogrammed if the FPGA has bitstream encryption disabled, or if you have the encryption key.
A certain vendor has locked down FPGA which require an encrypted bitstream. That makes non-Bitcoin resale value essentially $0.00.
Are you quite sure? My understanding was that a key was flashed into a non-readable and non-reversable memory area on the device, which could be consulted as necessary if the bitstream was encrypted. Otherwise, the key would be ignored.
Reagrdless of having a key in efuse or BBRAM, you may always use a non-encrypted bitstream through JTAG (see the configuration users guide, pp 83, paragraph 5.
Quote from: http://forums.xilinx.com/t5/Spartan-Family-FPGAs/Spartan-6-BRAM-AES-Key-Programming/m-p/194462/highlight/true#M14434