If the update would have failed then NASA would have lost means of controlling and communicating with Curiosity Rover.
NASA’s Mars Science Laboratory explains that the SW update does not come to fix any bugs. NASA actually wrote two different software for the rover. The first software contained instructions about the landing while the second software contains detailed instructions about the exploring mission. The hardware of the Curiosity Rover is modest at best. There was not enough storage space to store combined instructions from both realeased software.
NASA carefully planned this distant software update. Many fail-safes were added so that there was little to fear about the update. There still was the possibility that the unexpected happens. The update was done during four days. The software update team was very careful, methodical and paid attention to every detail during the update.
Curiosity Rover’ software update took in consideration many details. From the actual update period that could take days to Curiosity’s low power consumption mode during night and the morning reboot done to its computers everyday.
