What I found quite nice about 3.5e creature types and templates is that they distinguish between Features, i.e. HD, BAB, base saves and skill points, and Traits, e.g. low-light vision, immunity to critical hits, whether the creature eats, sleeps and breates, etc.
When a creature's type changes due to the addition of a template, it almost always gains the Traits of the new type, but not the Features unless specifically stated.
For example, the Fiendish and Celestial templates do not change any Features. Any template that changes a creature's type to Undead changes Hit Dice to d12, but only the Skeleton and Zombie templates change the BAB, base saves and skill points. The Half-Celestial and Half-Fiend templates only change the base skills points for racial HD, and the Half-Dragon template changes base skill points for racial HD and increases racial HD by one step, to a maximum of d12. However, the templated creature gains the Traits of its new type, whether Magical Beast, Undead, Outsider or Dragon.