ConnectionDefinition.java

@Designate( //
    ocd = Configuration.class)
@Component( //
    name = Connection.PID, service = { Other.class }, //
    immediate = true, //
    configurationPolicy = ConfigurationPolicy.REQUIRE)
public class ConnectionDefinition implements Other {
  
  @ObjectClassDefinition(name = "My Config")
  public @interface Configuration {

    @AttributeDefinition(name = "some config")
    long x();
  }
}

UpdaterImpl.java

@Component( //
    service = { Updater.class }, //
    immediate = true) //
public class UpdaterImpl implements Updater {
  
  @Reference
  ConfigurationAdmin configurationAdmin;
  
  @Override
  public void updateConfig(Params..) throws PortalException {
    // here we get the java.lang.IllegalStateException: Configuration Admin service has been unregistered
    final org.osgi.service.cm.Configuration configuration = configurationAdmin.getConfiguration(Connection.PID,"?");
     final Dictionary<String, Object> properties = configuration.getProperties() != null ? configuration.getProperties() : new Hashtable<>();
    // fill properties with overwritten values
    configuration.update(properties);
}

UpdaterImplAlternative.java

@Component( //
    service = { Updater.class }, //
    immediate = true) //
public class UpdaterImpl implements Updater {
  
  @Reference
  ConfigurationAdmin configurationAdmin;
  
  private Configuration configuration;
  
  @Override
  public void updateConfig(Params..) throws PortalException {
     if (configuration == null) {
         configuration = configurationAdmin.getConfiguration(Connection.PID,"?");
       }
     final Dictionary<String, Object> properties = configuration.getProperties() != null ? configuration.getProperties() : new Hashtable<>();
    // fill properties with overwritten values
    configuration.update(properties);
}