Embodiments described herein are directed to enabling a user to modify data stored locally during a data migration process in which the user's data is being transferred to the cloud. For instance, a synchronization service requests the user's device (e.g., a client-based server) to provide a hash value for each data object maintained thereby (including modified data objects) and compares the hash values to hash values generated for the data uploaded to the cloud. If a hash value provided by the server does not match any of the hash values generated for the uploaded data, this means that the data object associated with the unmatched hash value has been modified since the initiation of the data transfer process. The synchronization service receives the modified data object from the server and uploads it to the cloud, thereby synchronizing the data uploaded to the cloud with the data locally-maintained by the server.