I haven't seen any rules on this, so my group has been using the following 'house interpretation'.
When a high-level arcane caster casts Shapechange, they can assume whatever shape they wish at the stated HD limit. The HD or LA of the new shape have no effect on the caster's level or HD.
However when a template is added to the newly assumed shape, we ruled that the LA is deducted from the caster's level after the spell has been cast while the template is used.
If a 20th level caster wanted to become a titan vampire, he would first become a 20 HD titan. Then when he assumed the vampire template (LA+8), he would become a 12 HD titan. Since titans must be at least 20 HD, he would instead have to become a 12 HD hill giant titan. 12th level is not sufficient to cast the shapechange spell, but it had already been cast.
How have others interpreted this rule?
When a high-level arcane caster casts Shapechange, they can assume whatever shape they wish at the stated HD limit. The HD or LA of the new shape have no effect on the caster's level or HD.
However when a template is added to the newly assumed shape, we ruled that the LA is deducted from the caster's level after the spell has been cast while the template is used.
If a 20th level caster wanted to become a titan vampire, he would first become a 20 HD titan. Then when he assumed the vampire template (LA+8), he would become a 12 HD titan. Since titans must be at least 20 HD, he would instead have to become a 12 HD hill giant titan. 12th level is not sufficient to cast the shapechange spell, but it had already been cast.
How have others interpreted this rule?