Shape change comes in a variety of forms, including druid shape change, lycanthrope shape change, and the shape change spell. They vary in nature, and are not the same as polymorph (e.g. polymorph, polymorph self, monster polymorph).

See Death.

Being shape changed into a monster bypasses the “This does not apply to characters of any sort” line on DMG 75. Shape changing into a monster of sufficient HD will allow striking monsters that can only be hit by weapons with the corresponding plusses. ¹⁾

When a monster has a shape change ability, such as the vampire, it is not the same as the 9th level spell shape change. When monsters use an innate shape change ability, they retain their same defenses, vulnerabilities, and all powers which would not be made impossible by the new form. Any exceptions to this will be explicitly indicated in the monster description.²⁾