The first day of a two-day meeting of the council of the International Association of Athletics Federations (IAAF) saw the global governing body announce that the opening round of the men's 200 metres has been moved from day five (August 8) to day four (August 7).
Previously, the 400m final was set to start just hours after the 200m heats, with both races scheduled for late evening on August 8.
The 2017 World Championships will take place at London's Olympic Stadium from August 4-13.
IAAF president Sebastian Coe said he was "delighted" that the council had made it possible for Olympic 400m champion Van Niekerk, who at last year's Rio Games smashed American track legend Johnson's 17-year-old world record for the one lap event of 43.18 seconds by winning gold in a time of 43.03, to have a crack at the double in London.
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Johnson completed his 200m/400m double at the 1995 World Championships in Gothenburg.
Van Niekerk, who back in January said he was "definitely definitely doing the 200m and 400m in London", added that training with Bolt, a 100m and 200m runner, had encouraged him to think big, including running the first sub 43-second 400m.
"A window opened," he told The Times on Tuesday.
"Why not achieve what they can? When the clock stopped at 43.03 I had no choice but to think of going under 43. To dream of 43.02 would make no sense."
"This summer's World Championships are close to my heart. Twelve years ago London won the right to host the 2012 Olympic and Paralympic Games. That decision led to the building of the Olympic Park and the stadium and a decision to keep the track after the Games.
"This in turn meant that London could bid for and host this summer's championships."
Coe, whose time at the top of the IAAF has been dominated by the fall-out from revelations of state-sponsored doping in Russia, added: "I see a vibrant community space, the former athletes' village redeveloped into housing and a series of world-class sporting venues being used by Londoners on a daily basis and hosting international sporting events.