The switch of frontline seamers was the only change to a winning team, with all-rounder Ben Stokes declared fit after his knee injury scare as England sought an unassailable 2-0 series lead at the Ageas Bowl.
South Africa, on the other hand, have little time to put behind the loss and make a comeback to keep the series alive. That became 10 off 10 balls when David Miller (71 not out) struck Jake Ball for a six and a four off successive deliveries.
They shared an unbroken partnership of 62 off 41 balls, but that wasn't enough to spare Monday's match at Lord's from becoming a dead rubber.
But only singles followed as Miller and Morris ran out of steam.
Stokes took a full part in England's training on Friday, having bowled just two overs in the 72-run win over the tourists in the opening rubber on Wednesday at Headingley before leaving the pitch for treatment on a sore left knee. Stokes received reprieves off his first two deliveries.
Stokes smacked 101 - his second One-Day International century and joint-best score - off 79-balls as he punished the bowling in a blitz of big hits and aerial drives.
Stokes finally fell in the 44th over when he holed out in the deep to give debutante left-arm spinner Keshav Maharaj (1-72) his lone wicket.
He unfurled a range of destructive shots, sending Andile Phehlukwayo and Chris Morris for three massive sixes.
Next ball produced another edge, with wicket-keeper Quinton de Kock failing to hold a more hard chance.
But as Wednesday s match demonstrated, England can win well without a major contribution from Stokes, a point emphasised by fast bowler Liam Plunkett.