McCaffrey, 29, had been expected to come in for Samu, having replaced the Wallabies loose forward last week in the 38-13 quarter-final victory over the Sharks in Canberra.
McCaffrey, who had started the season as the first-choice number eight before suffering a knee injury, is the only change to the starting lineup from last week.
"The McCaffrey change is pretty smooth and there's no dramas there," coach Dan McKellar told The Canberra Times.
"They're different players, but they're both very smart footballers. Locky identifies space well on the field, he's clever and can be physical."
Samu scored two tries in the quarter-final but was replaced at halftime after he felt tightness in his hamstring and McKellar did not want to risk him with the travel to Argentina and short turnaround between games.
If the Brumbies win in Buenos Aires they will return to Australasia for the final to meet either the Canterbury Crusaders in Christchurch or Wellington Hurricanes at home.
Samu said he was targeting a return for the final if his team mates did the job on Friday in Buenos Aires.
"I'm hoping to be right for next week if the boys get the result," Samu told reporters in Canberra on Thursday.
"It's (the hamstring) a lot better than what it was yesterday, so I'm definitely improving and I'm just doing the best I can to get it right.
"The medical staff are really working hard to make sure I'm a chance."
Brumbies captain Christian Lealiifano, in his last season for the Brumbies before heading to Japan, will make his 150th Super Rugby appearance for the team in the semi-final.
Only former prop Ben Alexander (154) has played more games for the Canberra-based side than Lealiifano, who battled leukaemia in 2016 but returned to rugby the following year after treatment.
Replacement prop Leslie Leuluaiali'i-Makin will make his 50th appearance for the team when he gets on the field.
ACT Brumbies: 15-Tom Banks, 14-Henry Speight, 13-Tevita Kuridrani, 12-Irae Simone, 11-Toni Pulu, 10-Christian Lealiifano, 9-Joe Powell, 8-Lachlan McCaffrey, 7-Tom Cusack, 6-Rob Valetini, 5-Sam Carter, 4-Rory Arnold, 3-Allan Alaalatoa, 2-Folau Fainga'a, 1-Scott Sio.
Replacements: 16-Connal McInerney, 17-James Slipper, 18-Leslie Leuluaiali'i-Makin, 19-Darcy Swain, 20-Murray Douglas, 21-Jahrome Brown, 22-Matt Lucas, 23-Tom Wright.
(Reporting by Greg Stutchbury in Wellington; Editing by Ian Ransom and Sudipto Ganguly)
