Here's the deal with ur new phone..
1.- as stated before u cant downgrade, Samsung or Google Made the 4.3 so U cant Downgrade even thru Odin. (well u can but you won't get Rid of Knox counter and U will need to "void" your warranty by flashing a different version of android)
2.- You can still Root your phone without triggering Ur Knox Counter which since u are already have since you're on 4.3 and u cant get rid of it.. Just use CT-Root, debloat the phone and install whatever Root apps u enjoy (this is my current state since I too Had to exchange my phone)
3.- If you do decide to root your phone, only way to trigger the Knox counter is by installing a Custom recovery and Flashing anything thru it.
at this point it all depends on what u want to do with your phone, stay on Pure NOT ROOTED stock or Rooted Stock with no recovery or just go all out and root install recovery and trigger your Knox counter "loosing" ur warranty
Current Device: The Awesome Samsung Galaxy S4
Rom: all of them
Old devices: Samsung Galaxy S3
Sprint Epic 4G-Gave it away
Samsung Intercept Hard Bricked