The two options are have the different versions running on separate instances or have all versions in a single instance. The former lets you scale the versions

The two options are have the different versions running on separate instances or have all versions in a single instance. The former lets you scale the versions independently. If v1 traffic drops 5x overnight then you can scale it down by 5x. The latter simplifies the deployment and management though. You just have to deploy one service and scale one service to a single amount of traffic.