If they don't budge, it's possible that they've been glued in. Wilson actually recommends doing so. Just keep trying, they should loosen; even if a strong glue was used, leather will eventually strip. If they are really jammed, what will often happen is that the head of the screw will strip, and you can't use a standard screwdriver anymore. In that case you have to resort to other means: (1) if it's one of the raised round-head screws, first try a pair of Vice-Grips clamped tightly around the head of the screw [this won't work if it's one of the countersunk flat-head screws] (2) if that doesn't work, the method of last resort is to drill into the screw head and use a left-hand-thread screw extractor tool (alternatively, you can use a Dremel tool to grind the head off completely, but you will likely damage the heel plate a bit).
ETA: Something else occurred to me. If you try to unscrew the stuck screws to the point at which the heads strip, it's possible that someone was sloppy and got some strong glue between the screw heads and the heel plate: that is, the screws are now glued to the heel plate. In which case, you might want to consider slipping a large screwdriver in between the heel plate and the heel and prying the screws from the heel. Once you have enough clearance, place a wood shim between the heel plate and the heel so the screwdriver presses against the wood shim and doesn't gouge up the heel too deeply. If you do this carefully, the damage to the heel shouldn't be too bad and you should be able to repair it (assuming you are planning to do something else with the boots; if not, then it doesn't matter). Once you've removed the blade, you'll need to prop it up and clamp it so you can pound the screws loose from the heel plate (place a wooden block over the threaded end of the screw and strike the block with a hammer).
