They are usually glued on, well, at least the right hand throttle side is. When older the glue loses some of its adhesiveness, so they can generally be massaged off, though they may need to be cut. Putting on is a little easier, make sure the metal parts are clean and dry then spray the bars and inside grips with hairspray and quickly slide on. The spray will dry and grip really well.
BTW, high bars will also require using longer cables and brake hose.......considered that?
Bill..................;-)