Yeah I got it and I already completed FIRM downgrade. What I'm asking is if it's safe to perform system memory format after that. As Plaitect guide states:
I still have the Fieldrunners on my 3DS, so I want to minimize the risk of brick.
All variations of sysUpdater, not just PlaiSysUpdater, has a very small chance of causing a "soft brick" once the downgrade is complete and the device reboots. This will cause the system to boot directly to the Nintendo error screen instructing users to power off the device.
The reason for this is unknown, but can be prevented by performing a system format before downgrading. It seems to happen very often on systems with ambassador status or who have DSiWare installed.
I still have the Fieldrunners on my 3DS, so I want to minimize the risk of brick.