It seems the only Kerry/McCain counties that Obama won in the primary were the two Louisiana parishes that went Kerry to McCain--Pointe Coupee and Assumption. For these I would assume there must have been either very low white turnout in the primary or very low black turnout in the general (or both).
I haven't found any other counties that voted for Kerry in 2004, Obama in the 2008 Dem primary and McCain in the 2008 general. If anyone finds any others feel free to correct me.
I don't really think its turnout. Both parishes would tend to fit into the old school Dixiecrat type of counties. Both were trending Republican, but Kerry was able to get enough of the white vote to hold on to them. Both are around 30% African American, which likely resulted in a Primary that was around 50% African American.
Obama had virtually no support among whites in Louisiana so any parish which was 30% African American, probably was enough for him to win a Primary (since that meant the primary was about 50% African American) but African Americans making up 30% of the overall electorate wasn't enough for him to win the General