The pair were due to be on the ISS for only eight days, but because of technical issues with the experimental spacecraft they arrived on, they have been there for more than nine months.
The astronauts are due to begin their journey back to Earth later this week. Steve Stich, manager of Nasa's commercial crew programme said he was delighted at the prospect.
"Butch and Suni have done a great job and we are excited to bring them back," he said.
The astronauts, along with their ISS workmates, Nasa's Nick Hague and Roscosmos cosmonaut Aleksandr Gorbunov, will be relieved by four astronauts, from Russia, Japan and two from the US.
There will be a two-day handover after which the old crew are due to begin their journey back to Earth. But there could be a small further delay, as they wait for conditions on Earth to be right for a safe re-entry of the returning capsule, according to Dana Weigel, manager, of the ISS programme.
"Weather always has to cooperate, so we'll take our time over that if it is not favourable," she told reporters.
Ms Weigel explained that the astronauts had begun getting ready for the handover last week.
"Butch rang a ceremonial bell as Suni handed over command to cosmonaut Alexei Ovchinin," she said.
The astronauts have consistently said that they have been happy to be on board the space station, with Suni Williams describing it as her "happy place". But Dr Simeon Barber, of the Open University, told BBC News that there would likely have been a personal cost.
"When you are sent on a work trip that is supposed to last a week, you are not expecting it to take the best part of a year," he said.
"This extended stay in space will have disrupted family life, things will have happened back home that they will have missed out on, so there will have been a period of upheaval."
Butch and Suni arrived at the ISS at the beginning of June 2024 to test an experimental spacecraft called Starliner, which was built by the aerospace firm Boeing, a rival to SpaceX.
The mission had been delayed by several years because of technical issues in the spacecraft's development, and there were problems during its launch and docking on to the ISS. This included issues with some of Starliner's thrusters, which would be needed to slow the spacecraft for re-entry into the Earth's atmosphere, and leaks of helium gas in the propulsion system.
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