I would wait updating to 4.3 until we have a updated .zip with Knox removed. Most likely the 4.3 update has the Knox Boot loader. Checked the sprint forums and Knox is confirmed for them.
Here is some info about Knox that I got from the XDA's sprint galaxy s3 forum.
"What does voiding Knox mean? Boot loader flag goes from 0x0 to 0x1
FLASHING A: CUSTOM KERNEL or CUSTOM RECOVERY on MK3 boot loader VOIDS KNOX
CANNOT DOWNGRADE TO OLDER FIRMWARE!
Composed of an OS side and firmware (bootloader-side) implementation. Apps (OS components) can be removed, but the firmware components CANNOT.
You can still root on the new firmware...but you can't flash a custom recovery or custom kernel, or else you will trip Knox"