UFC officials announced the change on Tuesday.
Edwards (42-20-1, 1 NC) needs a win in the worst way. The 64-fight veteran has suffered three losses in a row. He was knocked out in his last bout against Yancy Medeiros in November 2013, but the fight was later ruled a No Contest after Medeiros tested positive to a banned substance following the fight.
Hallmann (14-2) made his UFC debut on Sept. 4, 3013 riding a nine-fight winning streak. He defeated Francisco Trinaldo by submission extending his winning streak to ten. The 26-year-old was defeated in his last outing by Al Iaquinta by unanimous decision.
UFC 173 takes place at the MGM Grand Garden Arena in Las Vegas on May 24. UFC Fight Night 42: Henderson vs. Khabilov takes place on June 7 from the Tingley Coliseum in Albuquerque. The event is headlined by former lightweight champion Benson Henderson and rising Russian Rustam Khabilov.