Bruce,
only one Da got a chopnose, went as a D/b-less heavy shunter to Tasman Forestry's mill, now retired and in private hands, is being rebuilt with Dynamics and a high short hood. 80 Da's were chopped and rebuilt with 645's into Dc, the D/b being moved with the Electrical cabinet to behind the cab. Some of the Db got the same treatment becoming Dbr, these got the 645 V-8 and had a charm all of their own.
Since the fiasco with the Chinese-built 3600hp loco's having asbestos paint and needing to be stripped/rebuilt, all the available older about to be withdrawn power is making its last stand. Great to hear at night for me at least, brings back memories of the good old days...
Wiki has each class at the bottom of the page, it does get involved but probably the only good source I know of.
Hope that helps.
Chris
in New Zealand