Class Level effect on a (Sp) ability

[Thomas LeBlanc]

Base Ability
Spell-Like Abilities (CL 1st)
1/day — doom (DC 10)

Spell-like ability has a caster level of 1.
A Cha modifier of -1
1 racial HD

Bestiary - The DC for almost all special abilities is equal to 10 + 1/2 the creature's Hit Dice + a relevant ability modifier (usually Constitution or Charisma depending on the ability)

I want to add 4 levels to the creature and replace the racial HD with class HD.

What is the new DC? Is it based on his new HD does it stay the same since it lists a caster level?

[IkeDoe]

Afaik most Spell like abilities for monsters in the bestiary have the standard spell DC (10+spell level+Cha modifier), the rule you mention is for Su and Ex special abilities.