This is really important to remember when you do not have the benefit of a “crop-factor” as DX cameras and other non full frame solutions give. When I moved from DX to FX (crop to full frame) this was the single most time consuming thing I worked on.
Croping or adding anything to this image would completely change the whole outcome of it.
I sold my D200 at the same time the D3 was included in my collection so there was no transition from one size to another. My 70-200mm 2.8VR was sufficient on DX camera but it was necessary to upgrade to a lens with longer range to be able to keep distance to the object. Taking pictures of things and sometimes people the distance between camera and object will be easy to adjust, not so with animals, most sport events and other situations.
Software may save a lot of the situation and a sensor with enough Megapixels will save a lot of situations but when the perfect shot apears, you want as many pixels as possible to cover the main intention of taking the shot. The details captured by an image sensor mixed with a perfect combination of the image processor can blow anyones mind, and impress any customer or photographer. However if you have a 15MP camera and the object covers 5 of those on a FX, it would cover about 6,65MP on a DX/crop. If you could be 15meters away with a DX, a FX will require you to be about 5 meters closer to fill the image sensor or put a larger lens on the body. On a pet this will not be an issue, however on a wild animal it may separate a perfectly sharp lions eye from a ever so slightly diffused one when enlarging it for print.
Those issues are pretty much habbits that needs a change when moving from one system to another, but take them into consideration. I love the FX variation on D3 and D800 and will not change back to a DX, but this is because I always make plans accordingly and that the full frame systems do have many advantages when in ISO and other functions. I will get back to the pros/cons of DX/FX later on in the development of this site, however do try to borrow or just try both system (i.e look up a local camera club or dealer) when considering changing from one to the other. Trying the two system with the same lens i.e. 50mm or 100mm on both, not 35mm on croped sensor and 50mm on full frame since this will not show the difference, may just help you to a better choice. After all there are so many options on camera accessories that the camera body itself have to match your needs and not limit the same needs.