1. A method for program conversion for a network system, comprising: creating a second virtual machine (“VM”) running on a network device in response to a program modification signal; building a second database having a second data structure in the second VM in accordance with requirements relating to a new version of software; accessing a first database having a first data structure in a first VM based on current version of software while the second database is being built; and switching over from accessing the first database having the first data structure hosted by the first VM to the second database having the second data structure hosted by the second VM when a switchover command is received from an in-service software upgrade (“ISSU”) manager.