Ahm... both fans blow in the same direction (they have to...) so switching the front and back fans would NOT result in one of the fans blowing the wrong way. All fans blow towards the side which has the label. E.g. if you are looking at the front of the miner, you will see the label on the fan and it will blow towards you. The back fan is ALMOST the same type (there is a slight difference) but it also blows towards the direction of the label, that's why you don't see a label on the back fan (it is on the inside, facing towards the guts of the miner).

If a fan blows in the wrong direction, remove it, turn it around, screw it back on, and it will blow in the right direction.

For those interested in the details, the latest S9s have slightly different fans front and back. I have no idea WHAT the difference is, but the front fan is NIDEC UltraFlo W12E12BS11B5-57 and the back one is NIDEC UltraFlo W12E12BS11B5-07 (note the last two digits). Both are rated 1.65 Amps and I see no visible difference between them. But if they took the effort to install two different fans in each unit (which means more complicated logistics), I'm sure they have a good reason for it. I can't find specs for the fans anywhere. If anyone has any info (or even a theory) about the reason for the different fans, I'm curious. Maybe different RPM at the same voltage to make it possible to feed them the same voltage and still maintain the slight pressure difference?