The path that leads a Human to turn into a Lich is long and tortuous, as much as the one leading from Beastman to Abomination and from Demon to Angel. It involves the collection of very rare resources, crafting of powerful artifacts to channel the magic, and Divine quests. It's not possible to transform back.
The final ritual of the Lich transformation sees the Human character join the embrace of Death to be reborn as a cadaverous, almost skeletal undead creature, with a whole new set of weaknesses and powers. Some are common to all undeads, such as weakness to fire, increased resistance to cold, resistance to poison and the need to feed oneself now gone. Others are more specific, such as a particular affinity for the Necromancy and Abjuration schools of magic. There will be differences in talent tree for liches, boost to regenerations, resistance to mental attacks, lose Constitution gain Intelligence, lose Charisma. Spells that affect Undead might also affect liches.
One final word here. You thought a Lich has an Evil alignment? Just because it’s not technically alive anymore? Bad! Prejudice! A Human turning into a Lich preserves its Karma and Alignment – it’s no different from a regular Human in this regard!
Liches will also get new looks. After completing transformation ritual you will be disconnected and in character selection menu you find your character with changed race. You will be able to finalize appearance of your character but not stats. Your stats will same except for gains and loses caused by transformation.