1. A system, comprising a plurality of computing devices comprising one or more respective hardware processors and memory, and configured to implement: a plurality of service managers implemented via one or more of the respective hardware processors of the plurality of computing devices and configured to coordinate a plurality of distributed, network-accessible services of a provider network, wherein each service manager of the plurality of service managers is configured to accept operation requests formatted in accordance with a supported operations specification of a respective service; and a metadata manager implemented via one or more of the respective hardware processors of the plurality of computing devices and configured to: receive an account state change plan request, comprising: (a) an indication of an initial account state of a particular client account of the provider network, wherein the initial account state includes a plurality of service attribute values of a set of services of the plurality of services in use by the particular client account, and (b) an indication of at least one configuration setting of a desired target state of the particular client account; generate a plan comprising a set of recommended operations to achieve the desired target state, wherein a first operation of the set of recommended operations is formatted in accordance with a supported operations specification of a first service of the set, and a second operation of the set of recommended operations is formatted in accordance with a supported operations specification of a second service of the set; and generate a response to the account change state plan request, comprising at least one of: (a) a representation of an operations sequence for the set of recommended operations to achieve the desired target state, (b) an indication of an estimated probability of success of the operations sequence for the set of recommended operations, or (c) an estimate of an expected billing amount associated with an implementation of the operations sequence for the set of recommended operations.