The amperage will not influence fan speed in the way you think. The fact that the new fan is rated to use less amperage means only that it uses less amperage and the motherboard will only supply as much as needed. Amperage is only the "strength" of the power and as far as I know the motherboard and PSU will keep this in check. If it didn't then the fan would probably have blown up by now as too many amps will make the fan's internal circuits overheat. Too much amps will even make the wires overheat and in some cases melt the wire. This is something many don't know when they do wiring on their cars and then wonder why it all went up in flames.
The voltage is what control the fan speed. The higher the volts, the faster the fan, and the more amperage it will pull.
Someone with more in depth knowledge could maybe explain more and better.
But honestly, this sounds perfectly normal to me. When the PC starts the fans will start at max and then settle down as the system starts getting temp readings.
