1. A system, comprising a processor; and a memory storing instructions that, when executed by the processor, cause the processor to perform operations comprising: implementing a network-accessible multi-tenant storage service for storage objects owned by a plurality of clients; receiving an indication of a lifecycle policy to be implemented for a collection of one or more storage objects, wherein the lifecycle policy comprises a representation of one or more lifecycle transitions of a particular storage object of the one or more storage objects and one or more transition criteria corresponding to each lifecycle transition of the one or more lifecycle transitions, wherein a particular lifecycle transition of the one or more lifecycle transitions comprises a transition from a first online service level with a first supported durability level to a different online service level with a second supported durability level, wherein the first supported durability level specifies a first maximum number of versions of the particular storage object, and wherein the second supported durability level specifies a second maximum number of versions of the particular storage object; determining, based at least in part on the lifecycle policy, (a) a data placement plan for at least a portion of data of the particular storage object in accordance with the particular lifecycle transition and (b) one or more modification operations to be initiated in accordance with the data placement plan; initiating the one or more modification operations; verifying that the one or more modification operations have succeeded; and providing, in response to a query, an indication of a current service level of the particular storage object.