D1 --> D1H (still have both) A very significant upgrade in the early days of pro digital bodies. Basically the same 2.7Mp sensor but menus, handling and IQ improved significantly. I love my D1H and still use it from time to time because of the gorgeous colours SOOC.
D2h --> D2Hs was a significant upgrade in terms of noise (I still have my D2H, searching for a D2Hs for collection purpose)
D2X --> D2Xs was .. some kind of an upgrade with some tech improvements and a bigger LCD
D7000 --> D7100 , just a trick to hide some troubles with the Original D7000 (*)
D600 --> D610, just a commercial trick to start selling initially crippled D600 bodies which were plenty of dust and oil because of a defective mirror system... (*)
D3 --> D3s was not just an upgrade
, but worth a new number. TMHO this camera should have been called D4. You can see my D3s vs D4 comparison here. D3s is still a great choise to buy today for photography (not for video though).D4 --> D4s, A new name for about the same
... (*) I need to mention that D4s seems to have some significant video improvements over D4.(*) = The above are just my thoughts and personal opinion after having talked with pro users who have both D4 and D4s or who did the upgrade, no statements whatsoever...
Kindest regards,
Stany