If Dlink says the downgrade is not possible, then it isn't. They've blocked it in the firmware writing routine. Most likely reason is that the newer firmware made a modification to the router that either cannot be undone or cannot reliably be undone. Most likely thing is that Ubicom gives Dlink a small area in the CPU to write microcode to address functionality issues. Depending on how Ubicom's development tools work, they may not support downgrading changes to the microcode.
Ubicom recently released a version of their development tools that supports Linux. Its also possible that these newer firmware versions are written using these new tools. Just a guess, but I bet I'm not far off....