MetadataPackageVersion

// Finds all Active subscriber orgs that have the package installed
String PACKAGE_SUBSCRIBER_ORG_KEY_QUERY = "Select OrgKey from PackageSubscribers where OrgStatus = 'Active' and InstalledStatus = 'I'";

// Finds all MetadataPackageVersions lower than the version given, including the list 
// of subscribers for each version
String METADATA_PACKAGE_VERSION_QUERY = "Select Id, Name, ReleaseState, (%s) from"
	+ " MetadataPackageVersion where MetadataPackageId = '%s' AND ReleaseState = 'Released'"
	+ " AND (MajorVersion < 3 OR (MajorVersion = 3 and MinorVersion < 4)"
	+ " OR (MajorVersion = 3 and MinorVersion = 4 and PatchVersion < 6))";

// conn is an EnterpriseConnection instance initialized with a ConnectionConfig object
// representing a connection to the developer org of the package
QueryResult results = conn.query(String.format(METADATA_PACKAGE_VERSION_QUERY, PACKAGE_SUBSCRIBER_ORG_KEY_QUERY));

// This list will hold all of the PackageSubscriber objects that are eligible for upgrade
// to the given version
List<PackageSubscriber> subscribers = new ArrayList<>();
for (SObject mpvso : results.getRecords()) {

		// Cast the sObject to a MetadataPackageVersion
		MetadataPackageVersion mpv = (MetadataPackageVersion) mpvso;

		// Add subscribers to our list
		if (mpv.getPackageSubscribers() != null) {
			for (SObject psso : mpv.getPackageSubscribers().getRecords()) {
				subscribers.add((PackageSubscriber) psso);
			}
	}
}