Dear, NGN will replace filnally all other types of networks with a converged packet based network. but this may take long to achieve this aim. so, in the transition phase, all legacy networks and equipment will be supported, even in the final network.
you can find more from ITU website (www.itu.int) and ETSI website (www.etsi.org).
there are a lot of tutorials on the net about ngn. just search with google for ngn tutorials.