Increacing HD size

[Bobmufin52]

I'm planning to apply the half-dragon template to a major NPC villain in a campaign I'm going to run soonish. I'm a little confused by its entry for what to do with its Hit dice though. I understand that I just make it the next highest dice available, but I'm not sure what effects this has on the monster (if any). Dose it increase its HP total? Dose it change anything at all? Thanks in advance for the help!

[Greenish]

If the villain has racial hitdice, increase those by one step (or grant him 1 hp per RHD). Otherwise, the hitpoints won't change.

[AslanCross]

Yep, it affects only racial hit dice. HD from class levels won't change.

[PlzBreakMyCmpAn]

Assuming the PC isn't already at D12 HD. If you need to reroll the HD (due to a template) you now roll the higher bolded value d4->d6, d6->d8, d8->d10, d10->