If the fans are the 3 wire type, NOT PWM 4 wire. You can switch the 12V+ with the GRND wire. On one side of the plastic connector there are 3 small holes you can see the terminals, in these holes you need to push them down with a small pin Don't use allot of force or you will smash the terminal and it won't stay in the connector when you put it back together the wires will pull out of the connector so you can switch them, this will change the Fan rotation. Most Fans will have RED = 12V+ BLACK = Ground YELLOW = RPM .

I did a double check and sorry But the fan I had was an old fan with just 2 wires something about the RPM wire prevents this from working so please don't wast your time with the 3 wire Fan.