re bios upgrade command is not valid on the srx320

Hi,

New SRX320 - after upgrade to 15.1X49-D50.3 trying to upgrade re bios (like on srx2xx)

> show system firmware
Part           Type             Tag Current Available Status
                                    version version
Routing Engine 0 RE BIOS        0   3.1     2.9       OK
Routing Engine 0 RE BIOS Backup 1   0.0     2.9       OK

1. Why backup bios 0.0 ?

2. Why Current 3.1 > available 2.9 ?

Attempt to upgrade via CLI - error

0> request system firmware upgrade re
error: command is not valid on the srx320

Attempt to upgrade via Shell - do nothing.

% bootupgrade -u /boot/uboot
%

So my questions are:

1. Do I need to upgrade or downgrade bios?

2. If yes then how to do that?

Thank you,

Dmitry.