I did that a while ago, so I don't remember for sure. I think I used a metal saw. Put the blade inside, reassemble the saw and cut both bushings at the same time. They may come out easily once you cut one side. If not, start again turning the assembly by 180 degrees.
You will also be able to press the new ones in place though you want to be careful not the damage them. Putting the new bushings in the freezer and heating the knuckle before assembly may help. As Joes said, you have good chances they will need to be rimmed. They may fit on the kingpin out of the knucle, but once pressed in there the pin won't fit anymore.
