You do NOT have to replace the LCA, but ^^ is correct, they need to be pressed in. If you want to replace just the bushings, remove the LCAs and take them and the bushings to a trusted shop or machine shop and they will press out the old ones and press in the new ones. Grease and re-install...done.
The Sti front LCA bushings will fit the 05-09 legacies, and they are (from what I hear) slightly harder/stronger than the stock bushings, and very reasonably priced. Do a search on here and you should be able to find the part ##.
The Perrin/Whiteline/AVO bushings are polyurethane, and will be an increase in NVH, although it isn't much in my opinion, and they all offer (I think) both stock caster and offset caster. I have the offset AVO and love them!