I went to replace mine a few weeks ago and I thought I'd have to drill them out - they were rusty and seized solid. Before I started doing all that though, I tried my "Impact" Screwdriver with a good fitting pozi-bit. I hit the impact screwdriver as firmly as I dared with a steel hammer and to my utter amazement, they started to move. It still took a while to get them all the way out, but it did work.
Now replaced with S/Steel Allen Bolts.