If you want to accomplish this (a totally worthwhile task IMO, since it makes SA stronger and spares Zimbabwe the horrors of becoming Mugabe's playground), a rather more manageable PoD is to have Southern Rhodesia vote 'yes' to the union plebiscite in the 20s.
Eurofed, you are my European history connection, you're great.
Early enough not to make Britain sanction South Africa to Hell and perhaps (optimistic guess) enough to have a less vicious Apartheid policy given the Rhodesians now in the population, might put the USSR harder to work in Africa though, a large, potentially strong and most assuredly pro-Western South Africa isn't going to be a game the Soviets want to play.
I'd guess that WWII and such would still happen, this is a butterfly that's HUGE for South Africa and Rhodesia but not immensely significant in international affairs, Hitler still seizes power in Germany, WWII still happens, OTL results largely are the same, the USA still falls out with the USSR, I can't see any reason to have it different.