Instead of manifesting your Doppel when your Soul Gem is full, you can now choose to temporarily transform your Soul Gem into a Grief Seed, allowing you to take the full form of your Witch. You may have become a Witch, but you still retain your mind and are fully immune to the effects of curses with no ill effects, and unlike with your Doppel, you don't lose Grief as you use this form. You can let go of this transformation at any time, reverting back to your Magical Girl form with a clean Soul Gem.
As a Witch, you can consume negative emotions from Humans or feed off other Witches to make yourself stronger. You can create Familiars which are miniature versions of you that work as an extension of your body. Familiars can feed off negative emotions to evolve into fully grown Witches. You can also create Barriers, pockets of separated space that reflect your despair, and a maze full of familiars to fight intruders. The size and complexity depend on your current strength and you possess a unique power based on your wish.
Alternatively, you can choose fully summon your Witch as an extension of yourself from your Soul Gem that you can control and use to fight.
Your starting power is based on how powerful you were as a Magical Girl. By having 80 or more karma you have enough Karma to completely rewrite reality to the size of an entire planet.