The UFC confirmed the change late Wednesday night.
The opponent switch was necessary after Pierson's original opponent, Brian Foster, was forced to withdraw after a pre-fight MRI discovered a brain hemorrhage.
The 26-year-old Ellenberger (24-5) has proven himself to be one of the rising stars of the welterweight class, winning seven of his last eight fights, including three in a row. Last time out, he beat Carlos Eduardo Rocha in a close, split decision at UFC 126. The April 30 date gives him about two-and-a-half months in between fights, but only about two weeks to prepare.
Meanwhile, Pierson comes in riding a six-fight win streak. The Ontario native made his UFC debut last December, defeating Matt Riddle by unanimous decision.
UFC 129 should make him a home favorite. It takes place at Toronto's Rogers Centre. The event is headlined by a welterweight title match pitting Pierson's training partner Georges St. Pierre against challenger Jake Shields.
As for Foster, a burst blood vessel in his brain is expected to heal on its own rather than require surgery. For now, no timetable is set for his return.