Exemplary methods for upgrading route processor virtual machines (RP VMs) and line card virtual machines (LC VMs) include upgrading a second RP VM serving as a standby RPVM with a new RP VM image. The methods further include spawning, using a new LC VM image, a second set of LC VMs to serve as standby LC VMs. The methods also include performing incubation of standby VMs, by configuring a switch to forward data traffic to both the first and second set of LC VMs, thereby allowing system states of the standby VMs to be auto-populated by the data traffic. The methods further include, after the incubation has completed, swapping roles of VMs at the virtual router, such that data traffic are exchanged only between the switch and the upgraded VMs.