Swap out the pins in the fan only, never the PSU. I once changed the pins around on a double up psu four pin lead and in my haste connected it to the HDD instead of the fan. One Quantum paper weight later I'd learned my lesson. Also mark the fan connector with a permanent marker pen as to the voltage mod so you always know in the future.
If you go by the above link and follow my tip it will work fine. I have two 80mm's running at 7v ATM.
[This message has been edited by BladeRunner (edited 06-20-2001).]