I can't find if there is an official ruling on, what happens to a creature types immunities if a creature gains a template?
For example, if a Giant Black Widow Spider gains the Boreal template, then it's creature type changes from Vermin to Magical Beast. The template states, "do not recalculate its Hit Dice, base attack bonus, saves, or skill points."
When it was the vermin type, the spider had the Mindless trait. Which gave it no intelligence score and immunity to mind-affecting effects. Does that mean the spider loses this trait, suddenly gains 10 intelligence, three feats since it has five Hit Dice, etc?
Similarly, if an undead gained say, the Half-Fiend template, it would become a native outsider instead of an undead. Would it lose it's immunities, gain a constitution score (presumably a 10), base it's hit points off that new constitution score, be healed by positive energy, etc?
1. The black spider remains mindless unless the template specifically changes this.
2. You be undead, no different than applying the zombie template to a tiefling. The half-fiend template would be something the creature had before dying, as it can only be applied to a living creature. The native outsider type would change to undead.