Just wanna address this, because I think it's important. Canon DID change their lens mount a few years ago, which made all lenses prior to the new camera range redundant. A LOT of photographers have never (even today) forgiven them.
Conversely, Nikon have never rendered any old lens useless. The oldest lenses made by Nikon go straight on the front of my camera. The oldest need a simple and inexpensive AI to AI-S conversion, simply because of what's generally accepted to be rectifying a poor design decision in early lenses. My favourite lens of all time, and one I still use today, a 105mm F/2.5 was introduced to the world 7 years before I was. I picked it up for about £30 ($45).
Backwards compatibility is a core feature of Nikon's pro and semi-pro series cameras, and there are even M42 mount converters available for the D40 consumer-level camera, so I feel it necessary to defend them.