Upgrading restrictions and guidelines
S.15.09.0029 uses BootROM S.14.03. If your switch has an older version of BootROM, the BootROM will be updated with this version of software.
Since S.15.09.0029 is the very latest (and last) firmware version for this switch, then it means that BootROM version S.14.03 will also be the most current and last bootROM version.
With the commands you have listed you are installing firmware image (not BootROM) S.14.41 to the secondary flash partition and booting the switch from this partition.
Primary and secondary flash are partitions which can store the same type of firmware images. It is not primary for firmware and secondary for bootROM.
Please note that the versions of firmware and bootROM dont have to match. BootROM is not updated so frequently as the firmware.
Also there is no procedure to only update the bootROM. The BootROM will be updated automatically with some firmware updates, thus some firmware images will contain bootROM update. If this is the case the switch will automatically reboot twice. First the update the bootROM and then to update the firmware. So no user action is necessary.
In your case you have the latest version of the BootROM so nothing to worry about.