Wikipedia has an explanation for staying with the 8 engine config. http://en.wikipedia.org/wiki/Boeing_B-52_Stratofortress#Engines
Basically cost to do the upgrade and upgrade the tools, training, and so forth in the supply chain.
One of their arguments is it will reduce global warming so they must be grasping at straws.
The reason I ask: if the B-52 was expected to be retired by now, replacing the engines back then would have been a questionable effort. But, latest extension would have resulted in a major cost savings, especially given the increases in the price of jet fuel.
The proposed engine was used in the 747, 757, and 767. So, it would still be in the supply chain.