If the motor speed tracks the supply voltage, the power consumption would change with the cube of the speed change ratio.
However, the datasheet gives a fixed 3.6W power consumption so the motor may be regulated to a fairly constant speed.
In that case, current could decrease as voltage increased, to maintain the same power.
Or it could be somewhere inbetween those extremes...
Why don't you just test one? As you're planning to operate it beyond it's specified limits, that's probably the only way to know for sure.
Also, be sure to test it under whatever transient conditions that your 30 Volts may feed it. While I would expect the fan's absolute max ratings (which the datasheet doesn't give) to be at least a couple of volts above its sec'd nominal, you're using up a fair percentage of whatever safety margin that might have been.