Saints general manager Mickey Loomis confirmed the moves Wednesday.
Gramatica, who had been kicking with a sore groin, has missed four field goals in the past three games. All of the misses, one of which was blocked and returned for a touchdown by Minnesota on Monday night, proved critical in close losses to Denver and the Vikings.
Gramatica, signed by the Saints late last season, beat out Mehlhaff in training camp and was 8-of-8 on field goals before the Saints' 34-32 loss at Denver in Week 3, when the first two of Gramatica's four recent misses took place.
Mehlhaff, a rookie from Wisconsin, has yet to attempt a regular-season kick in the NFL.
During the preseason, Mehlhaff made two of his three field-goal attempts and all four of his point-after attempts.
The change in kickers was one of several moves announced Wednesday by the Saints, who've been plagued by a slew of injuries that have knocked several starters out of action, including defensive tackle Sedrick Ellis and cornerback Tracy Porter.
Ellis is out for up to a month following arthroscopic surgery to repair a torn medial meniscus in his right knee. Porter dislocated his right wrist during a loss to Minnesota on Monday night and was placed on injured reserve Tuesday, ending his season.
Also placed on injured reserve was backup fullback Olaniyi Sobomehin, who injured his shoulder when he absorbed a heavy hit and fumbled on the goal line during the Saints' Week 4 victory over San Francisco.
New Orleans also made a pair of changes to its practice squad, releasing safety David Roach and linebacker William Kershaw and filling their spots with cornerback Darrell Hunter and linebacker Pago Togafau.