If you're rooted and want a stock rooted 4.3 install, I highly recommend root66 thread. Stock installs are best done from scratch because modified system/app can stop the update or cause issues.
If you have a custom stock ROM you can just flash 4.3 ROMs from recovery. (TWRP, CWM)
I'd stick with the current boot loader you have if possible in case you want to downgrade later on.
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