Some spellcasting classes (e.g. Sorcerer or Eldritch Knight) have the ability to 'forget' one of their known spells and swap it for another when they level. If they knew Find Familiar, and used it to obtain a familiar, can they swap out the Find Familiar spell but keep the familiar?
Obviously they would not be able to change its form, or replace it on death, since either of these abilities requires casting the spell again. But can they keep the familiar with all its abilities?