Advancing Monsters and CR

[Diarmuid]

The MM has the tables for advancing creatures by type and then has another table for determining how many HD added determines when the CR should be bumped.

What happens if you advance a creature but dont hit that CR bump level?

Take the Worg for example. They're 4HD, and have advancement listed as 5-6HD stays Medium, while 7-10 becomes Large. As a magical beast, adding 3HD would bump the CR, what does adding 2 HD do to the CR?

WRT to the advancement itself, if a creature has a single attack listed...if it's BAB becomes 6 or higher, does it gain interatives with that single attack?

[Keld Denar]

DMs discression at that point. A single foe with 1-2 extra HD might not cause a noticable CR bump. 6+ foes with 2 extra HD might warrant "effective" CR of every other worg increased by 1 and increase the EL of the whole encounter by 1. There is no hard or fast rule there. Thumb it up and call it good.

And no, natural attacks never gain iteratives. Only manufactured weapons and Unarmed Strikes can ever make true iterative attacks.