Aight guys, I bring you good news: You can rebirth as an abomination using old saves, and I strongly encourage you to rebirth in general because even though you can load old saves, the update breaks the Dark Cauldron skill.
Further good news, it looks like you can choose to rebirth as a human even if you've given birth to Deep One and Abomination babies. The problem is that the all of them are listed as "human" but the last option seems to be Abomination. Here's a screenshot:
Abomination Lona starts with the debuff "Sickly" which lowers base health and stamina.
Edit: Nevermind, you just need to talk to a the one abomination NPC in Noer. You gain two skills, but it would seem that there is no transformation. The skills have different effects based on whether you're tapping or holding the key. The first one lets you rubber band towards wherever you're attacking or yanks a target towards you. The second lets you summon a monster from a corpse or eat the corpse Prototype-style.