I have the SX830, and will be using the SX6xx series for an upcoming machine. Fantastic cases, though not quiet. My dual-athlon box has 7 fans (PS, 2xCPU, Geforce3, and 2xcase). Sounds like an airliner! But the latched door, drive rails, and removable HD bay are now must-have features for me.
Anyway, no need to move the case fans. As I see it (and I could be wrong here, but it's working for me), if you keep the cover on (which you should), the rear and PS fans will promote airflow from front to back -- they will all be sucking air from the front. If you get one of the fans blowing, it could upset this natural airflow -- air can now enter OR exit through the empty rear fan slot -- and can't increase the total air flow.
To see what I mean, keep the standard fan config and turn the machine on. Put your hand on the front grille; you'll feel cold air being sucked in. Put your hand over the rear fans; you'll feel warm air being blown out (nowhere else to go).
In theory, you could install more fans at the front blowing in, but I don't see the point. The rear fans are already exchanging as much air as they can. Using the HD fan slot might be helpful to promote air flow over the HDs, but it's not going to get more total air flow into or out of the box. To my mind, it seems most useful to promote a steady stream of airflow from one side of the case to the other, bringing cool air in one side and expelling warm air out the other.
Terry