1. A non-transitory computer-readable storage medium having stored contents that cause a computing system to at least: instantiate, by the computing system, a virtual computer network having multiple computing nodes at a geographical location based on configuration information received from a client, wherein the instantiating includes overlaying the virtual computer network on a substrate network at the geographical location, and wherein one or more computer systems that host the multiple computing nodes are co-located at the geographical location and are configured to forward, based on the configuration information, communications over the substrate network between the multiple computing nodes at the geographical location; and update, by the computing system and while the virtual computer network is in use, the configuration information based on instructions from the client to add functionality for an indicated computing node in the virtual computer network at the geographical location, wherein one or more computer systems that host the multiple computing nodes are configured to forward, based on the updated configuration information, an additional communication over the substrate network to the indicated computing node to obtain the added functionality for the additional communication.