If you want to keep the sleeves, i.e. you're using stock style bushings, you're in for some fun

OTOH, you really *really* want poly bushings. Just a set of greaseable polies will increase flex considerably, now and for the future
See
http://www.offroaddesign.com/catalog/greasebushings.htm
If you are putting in aftermarket bushings, i.e. ones that don't require the the sleeves, then go to town. If you have one, the oxy-acetylene torch is the fastest motivator, as you just melt the rubber out (it smells BAD, btw) and then chunk out the sleeve.
If not, careful drilling is in order. I say "careful" as breaking a bit off is easy, and makes it even harder to get a clean cut through the thing. BTDT. Once you have a decent-sized hole, you might use a small sawzall blade (like a jigsaw) in there.
I remember messing about with a propane torch and just making a stink, then switching to the drill and eventually, brute force with a chisel (aka the big flathead screwdriver) and a small sledge. Then a bit of cleanup with the Brillo pad on the inside of the eye ... but then you think about a coat of paint, and maybe some teflon or UMMHW tape... it snowballs
Anyway, that's the other great thing about putting in the poly bushings ... you can beat the living snot out of the old ones, and as long as you're careful not to score the inside of the spring eye, the new ones just sliiiiiiide on in
-- A