The following ability becomes available to warlocks with a specific patron at level 3:
By sacrificing one spell slot, the warlock gains the innate ability to exchange their current form for their other form. In so doing, their current form becomes their other form, and vice versa. Their current form, whichever that might be, is always their true form. The warlock’s other form must be the same size as their current form, and is the same kind of creature as their current form, but the appearance of the other form is otherwise freely changeable.
This ability can be used twice, and then requires a long rest before it’s used again.
The other form is fixed, but can be changed in communion with the warlock’s patron. In communion with their patron, the warlock can also renounce their other form to regain a spell slot.