Is the new line of training cups (which went on sale in August 2005) interchangeable with the previous series?
They are not interchangeable, so you should use those within the same series. But you can attach the Peristaltic Nipple Y (Wide-neck Peristaltic Nipple Y-Cut) from the new series to the Baby Cup and the Training Spout Cup from the previous series. Furthermore, you can attach the Peristaltic Nipple and Training Spout from the previous series to the Baby Cup from the new series.