Vulture reports that the two prime-time comedies will be replacing the Vanessa Williams drama 666 Park Avenue.
The supernatural series about a New York apartment building possessed by a demonic force debuted in September, but steadily-falling ratings resulted in its cancellation.
Happy Endings and Don't Trust the B----- will now air twice a week for the next several weeks on both Sundays and their regular time slots on Tuesdays.
Both comedies have faced uncertain futures on ABC, with reports of the possible cancellation of Happy Endings coming out earlier this year.
The new change is scheduled to go into effect on January 6, 2013.
Watch a trailer for season three of Happy Endings below: