Encontrei uma resposta na página 192 do PHBII em Reciclagem de recursos de classe.
Some class features offer two or more different options, such as the choice of combat style a ranger must take at 2nd level. Class feature retraining allows you to swap out one option for another [...] Upon gaining a new level, a necromancer could change her school specialization to evocation, thus becoming an evoker. At the same time, she could also choose to change her prohibited schools from conjuration and illusion to abjuration and transmutation. Doing so would cause her to lose access to all spells from the newly designated prohibited schools. Even if her spellbook contains those spells, she would lose the ability to prepare and cast them.
Além disso, estou ciente de que os liches não precisam de Necromancia, mas o debuffing é crucial para mim. Clone também. Realmente as únicas duas magias que eu uso regularmente são Mirror Image e Invisibily, as quais podem ser transformadas em scrolls e usadas com UMD.