You can always flash back to stock using ODIN but once you trip Knox it will always be tripped. Of course, I believe Samsung is the only one that can reset your knox counter.
Even Samsung can't reset a Knox counter. Once e-fuse is tripped it would require a new efuse, necessitating a new main board.
Once tripped you lose ALL hardware warranty, Samsung won't replace your main board unless
1. You get lucky and another part of your device dies such as CPU / GPU
2. The technicians replacing that part do not spot Knox has been tripped - very unlikely
3. You return to your carrier and they replace it without checking - though many carriers now insist on repair and then your back to 2.
Essentially Knox is the reason I resist rooting or flashing other roms. Especially as I resell and people always want some warranty left on the device and I couldn't in good faith sell something to someone with banjaxed warranty.