Speedfan may be able to change the voltage going to the various fan slots and thus change the speed, but it won't always work as it'll only support what it's coded to - plus, it can be fiddly to get it set up just right. Your BIOS may or may not have support for fan speed adjustment, sometimes you're able to change the fan curve there though - meaning you can have it automatically adjust the speed based on a temperature reading, such as your CPU or chipset temperature.
Two questions - firstly, why do you want to change your fans? By which I mean, what do you want to get out of the swap? The Antec Tri-Cool fans are pretty decent as standard, I wouldn't consider sickle flows an upgrade. Are they too loud, do they not push enough air, or something else?
Secondly, do you anticipate buying fans that run fast enough to be considered loud, slowing them down, then having them speed up again under load? Your system shouldn't need this, I would consider just buying some fans that are quiet as standard, and just run them at full speed or a fixed speed via a resistor.
If you really want fine grained fan speed control, you can't beat buying a proper fan controller. This will typically sit in a free 3.5" or 5.25" bay and give you dials to adjust the speeds of fans connected to it.
