Innate Spell Evolution (3.5e Variant Rule)

Innate Spell Evolution
Whenever a character with a spells known progression gains new spells known, they may "evolve" up to two existing spells known to fill the new slots. Some conditions must be met for this to occur. The lost spell must be 1 or 2 levels lower than the gained spell. The spell gained must belong to the same school and subschool as the lost spell. The descriptors of the new spell must be the same or extremely similar to those of the lost spell, though this may be accomplished with the assistance of feats such as Energy Substitution if the spell is learned with the feat applied by default.

If the new spell meets the conditions above the old spell known is lost, having evolved into and been replaced by the new spell known. The spellcaster may select any spell normally available to them to fill the vacated lower level spell known slot. The selected spell must be of the same level as the spells known slot, of course.

This innate spell evolution occurs in addition to the normal spell retraining allowed for the classes.

Examples
A sorcerer summons things for his shtick. He currently knows summon monster I and summon monster II. When he reaches 6th level, he elects to evolve summon monster I into summon monster III. As this meets the conditions, he gains the higher level version, loses the lower level version, and may select a new level 1 spell for the vacated slot. He could have instead evolved summon monster II, but decided that he liked having a second level summing spell as a backup.

Had the same sorcerer been into necromancy instead, he could have evolved cause fear or scare into fear.