A customer submits a set of preferences including hardware specifications for instantiating a virtual machine instance. The virtual machine instance may be instantiated in a physical host computer system that does not conform to the set of preferences. The virtual computer system service may be configured to evaluate the specifications of the available physical host computer systems to determine whether any of the available physical host computer systems conform to the set of preferences. Accordingly, the virtual computer system service may further evaluate the available physical host computer systems to determine whether available physical host computer systems comprise available slots for instantiating the existing virtual machine instance. If the one or more available physical host computer systems have available slots, the virtual computer system service may migrate the existing virtual machine instance to an available physical host computer system in order to fulfill the customer request.