You can update the firmware to an older version of the firmware if you have a copy somewhere. I'm not sure where that would get you.
The partitions that hold the software you'd need to replace have been locked since the dash shipped. When Sony did updates, they recomputed the signature for the updated software so they stayed locked.
The "/psp" partition was not locked since it was designed to hold configuration files, so there was a hack for the next-to-last firmware release that allowed you to add entries to the crontab that could execute a binary, but it seems Sony closed that hole as part of their 1.7.1526 firmware release.
Yes, it would be nice if Sony were release a new bootloader that would not enforce the security, but it seems highly unlikely to happen at this point.
Fortunately, the method we're using to get into the device does not require the software to be modified.