I drive out the front sight pin first with a pointed punch then witha parallel punch then drift out the old sight.
I Carefully drift in the new FS.
With the assembly in a vice. I hold the FS in place with a dowel and use a No 49 drill bit in the dremel and push this carefully though the fs hole, whilst holding the sight in place with the dowel. The with a flat punch punch the pin back into the hole.
make sense?