1. A computer-implemented method for migrating a virtual machine instance, comprising: under the control of one or more computer systems of a virtual computer system service, the one or more computer systems configured with executable instructions, receiving a set of preferences from a customer, the set of preferences including one or more hardware specifications for instantiating a virtual machine instance; instantiating the virtual machine instance in a first physical host computer system, the first physical host computer system comprising hardware that does not conform to the set of preferences; detecting available capacity within a second physical host computer system, the second physical host computer system comprising hardware that conforms to the set of preferences; and migrating the virtual machine instance from the first physical host computer system to the second physical host computer system partially due to having detected the available capacity within the second physical host computer system