Only possible via custom ROM. Cannot happen with stock tars through Odin. Downgrading the entire software with bootloader is not possible thanks to Knox.
For example....I'm on sac23 custom 4.4 v3 NAE and I would love to try out a few of the 4.3 custom Roms available. I can run these? I don't think I'd go back to stock 4.3...if its stock...its gotta be kk!