In a game that I'm in, the DM has decided to go with a lycan vs. vampire concept, and this has created a few conundrums.
One of the characters in our group is a dragon disciple, and another character is a barbarian. Both of them decided to take the vampire route, but no one is sure what happens with their Constitution-increasing abilities. For example, when the barbarian rages, would that increase his Charisma, instead of his Constitution?
So far as I can find, whenever a creature doesn't have an ability score (such as a construct or an undead or an incorporeal), generally they use another ability score for all purposes the other would cover. For example, undead use their Charisma modifier to determine health and fortitude bonuses, as they don't have a Constitution score.
When a creature doesn't have an ability score, and that ability score receives an increase from an ability/spell, how does that affect the creature?
Related question for 3.5e: Constitution bonus to HP for a necropolitan
Best Answer
Constitution bonuses aren't converted to Charisma.
As okeefe points out, undead in Pathfinder use Charisma to determine hit points, saves, and special abilities related to Constitution. However, that rule does not mention anything about replacing bonuses to Constitution with bonuses to Charisma. If you have a non-ability in Constitution, then you simply don't gain the effects of that bonus.
Also important to note, from the undead subtype:
And from the Barbarian Rage entry:
Note that undead are specifically immune to morale effects, and the Barbarian's Rage gives morale bonuses. In this case, regardless of whether or not the stat boost to Con shifts to Charisma, undead gain no benefits from raging. You may want to talk to your DM about houseruling some of this stuff if he wants your Barbarian friend to keep being effective.