The 5e by Roll20 NPC sheet does not include auto-calculations for many fields. This is because there is no standard calculations for NPCs (5e decided to ad-hoc the construction of NPCs rather than use a formula). HP and AC are two of the fields which are not autocalculated, sorry. My suggestion is to make a note on the character of what gets leveled when the character gets leveled. With all of that said, you are a Pro user so a Mod (API) script could do this. If one of the existing Mods won't do that then I am sure an API scripter should be able to whip one up easily enough. I would post a request for that in the Mod forum.
