As already said, they became Defenders about 1990, specifically when the Tdi engine was fitted. It involved major changes, the interior light came on automatically when you opened the door.
However Land Rover then applied the name retrospectively to all Nineties and OneTens, for example the parts book is titled Defender even when it covers far earlier vehicles.
As regards the changes, there were just as significant changes both before and after the name change, so identifying the vehicle by year/vin no is far more important that the name. Bear in mind many Defenders have been modified, so don't be surprised if the "correct" bits are don't fit!