Even if they make such a comment on a difference in AC or other properties, your players are likely familiar with how easy it is to hit zombies and other undead, much easier than their living counterparts in general, and with how mindless and instinct-driven they can be. In my experience, having used a similar stat block swap on many occasions, the most that players tend to notice is a difference in AC, and but usually only if they happen to also have DM experience. If you describe the revenant's appearance as a recognizably dead and rotted version of the former orc that discarded its weapons to mete out vengeance with its blood-encrusted fists, you'll get player buy-in without having to make any changes to the revenant stat block whatsoever. You'll know the difference, but your players aren't psychic, even if we as DM's are often paranoid that they are. Unless you let your players read your monster stat blocks during combat, they will likely not notice the difference between the old and new stats for this enemy if you use the revenant stat block as-is and just convince them it's the orc war chief through the narrative. There is no revenant template, but I don't think you should worry about it. There is no template, but just use the revenant.

Dnd 5e monster template