Those Eastern European migrants might actually be a mixed blessing for the regimes.
The vast majority of those migrants are people motivated by a need for a better standard of living. They won't have the same animosity Afrikaners have for native Africans, and might actually become agents of change.
Not sure, post-ww2 British migrants in Rhodesia quickly got the racist attitudes of the pre-existing urban population, I'm not sure about all post ww2 Migrants in South Africa, but I know that Portuguese Mozambican "retornados" (can they be called like that if they didn't return to portugal?) who went to South Africa after the carnation revolution were among the staunchest supporter of Apartheid and of the National Party in the 80s.