Who replaces them -- I'll go with the obvious on Britain & the US:
The Brits will have Atlee succeed Churchill. He was already well respected, involved in the coalition government, and became PM in 1945.
The US succession is also obvious -- Wallace will take over, but I don't see how he will get ELECTED to the office in '44. It might prove to be an interesting election with Wallace's suspected Communist ties and leftward leanings.
The USSR is the hard one. Trotsky is dead and deported at this point, maybe Malenkov or Beriya would be good candidates. If I were a betting man, I would say the latter rather than the former.
How will all this effect the war? Beats the hell out of me. You could set up a scenario in which Soviet agents actually want a delay in D-Day so they can end up with more of Europe. Likewise, it could go the other direction with D-Day being moved up to help the Russians only leading to a better parceling of post-war Europe in the West's favor. You might have the confusion of 3 governments in the US inside of 2 years leading to inconsistent foreign policies that, in turn, create a more favorable world for the Soviets and less animosity coming from the other side of the Iron Curtain (which won't have that name!).
Let the tearing apart of these suggestions begin.
J