In most cases the front (usually intake) also has some sort of mesh for filters, while the rear (usually exhaust) do not. If you do this, you probably want to ensure you have filters for the rear. another consideration is the front intake is usually low in the case (cool air sinks) while the rear exhaust fans are usually higher (hot air rises). also, you cpu HSF usually is oriented to blow toward the back toward the exhaust. Things to consider if you change this typical orientation.