If you have an early model Graphite 600T?; The front fan is held in place with 2 separate plastic mounting brackets. The 600T in the pictures is a later model Graphite 600T, which is why those pictures look different to yours.
Open your case, look at your front fan from the rear of the case, you'll see the plastic brackets on the left and right side of the fan. At the top and bottom of each bracket there is a screw that holds the brackets to the front of the case. Unscrew these, then fan AND the brackets will detach from the front of the case.
Because of these brackets, you can only install a front fan with the dimensions of 200mm x 200mm x 20mm, (i.e., the fan can't be any thicker than 20mm).
Most 200mm after market fans are 200mm x 200mm x 30mm, and obviously won't fit. There are some available in 200mm x 200mm x 25mm, depending on which one, in the past I have read of some people managing to install one this size with a small modification.
Otherwise, you'll have to find a fan that is 200mm x 200mm x 20mm if you really want to change your front fan, or modify the front of your case with a drill and Dremel to allow you to mount a thicker 30mm fan.
The 200mm fans fitted to the original model Graphite 600T also had a slight redesign and upgrade, and were first seen in the White 600T SE. Soon after, they also changed the Graphite 600T to the same design as the White 600T SE.
I hadn't had my Graphite 600T long when my front fan began to fail. I raised an e-mail with CS explaining the problem, and also discussed with them the difference in fan design between the early model Graphite and the White 600T's. CS very kindly sent me a pair of the newer spec'd fans from the White SE free of charge, and they arrived in 48 hours upon request from CS USA to the UK :).