Hi,
You have already examples in mob_skill_db.txt, like beelzebub mob:
When the HP is 50% (myhpltmaxrate,50), it will change from the ID 1873, to the ID 1874.
1873,Beelzebub@NPC_METAMORPHOSIS,chase,193,1,10000,5000,5000,no,self,myhpltmaxrate,50,1874,,,,,,
1873,Beelzebub@NPC_METAMORPHOSIS,attack,193,1,10000,5000,5000,no,self,myhpltmaxrate,50,1874,,,,,,
There're 2 of them, one is when is attacking, the other one when is chasing a character.