Regarding why DARPA changed their name, it was because of a change in focus. Originally they were much broader and were also involved with early projects relate

Regarding why DARPA changed their name, it was because of a change in focus. Originally they were much broader and were also involved with early projects related to space and nuclear test monitoring, among others. Around 1970 ARPA was consolidated and these responsibilities were transferred to other agencies (NASA and NNSA in the cases that I mentioned), with ARPA then focusing mostly on defence related research and becoming DARPA.