In this example, I am trying to calculate the CR of a black pudding [CR 7] that has its HD raised by 5 [Originally 10HD, now 15HD], deals a 5-10 foot AoE splash acid damage whenever damaged via bludgeoning [DC 21 reflex negates], but it raises a good question on whenever I add hit dice to creatures beyond their original starting point, but below their threshold [i.e. 15 HD black pudding] is it standard to add to the base CR of the creature, or are there rules saying that if it isn't above its threshold of HD, it would retain its CR.
If this matters at all, the levels 15-15-14-13 party recently fought the ooze and are confused at the paltry experience.