Jennings, the knobs slip right off. If you still have the stock stereo chances are that your HVAC knobs just need a little tug to come right off.
Becareful though, after i had my stock stereo replaced, the knobs have been loose. Some more experienced members may know why though.
from the looks of it, once the plastic knobs are removed, what kept them in place was some sort of clasp that keeps the knobs firmly in place. But once you remove them the clasps or whatever holds the knobs in place loose their grip.
My 2003 has gone through 2 stereos, mainly to upgrade to AUX/USB and HD car stereo units. I haven't found a solution to keeping the knobs firmly inplace other than to superglue them which isn't an option.