Spell Specialization says:
Select one spell of a school for which you have taken the Spell Focus feat. Treat your caster level as being two higher for all level-variable effects of the spell.
Every time you gain an even level in the spellcasting class you chose your spell from, you can choose a new spell to replace the spell selected with this feat, and that spell becomes your specialized spell.
Does the feat only matter for the school or does the subschool matter as well?
Example: I have Spell Focus Evocation. If I use Spell Specialization on Magic Missile, school Evocation [force], can I switch it to Fireball, school Evocation [fire]?