If zombie is in a Necromancy zone, would giving this Con save Advantage increase CR?

@AndrewF_WriterZombie in MM has Undead Fortitude. If one is in a Necromancy zone, would giving this Con save Advantage increase CR? it's probably not a big enough shift to increase CR – CR also accounts for attacks and damage #wotcstaff

— (((Mike Mearls))) (@mikemearls) October 19, 2016