i would definitely recommend removing the heatsink to swap the fans since you dont want to be torquing on the heatsink trying to get the fan detached.
Then I'm going to need some very detailed instruction on how to do that. I don't even remember how I got it on the first time, except that I had to get this mounting clip that gets glued to the back of the motherboard (sold separately from the HSF of course), and even then it was an enormous pain.
And will I need to re-apply thermal compound when putting the heatsink back on?
just a question: will you have your heatsink oriented to blow air up, or horizontally?
Horizontally. The case is an Antec Solo, and has vents in the back, but not on top.
And shouldn't the fan's speed be controlled by the motherboard? I'm pretty sure that's what my current one is doing.