I have done quite a bit of this, and continue to rework trees every year, as I learn which varieties perform well here. There are some good YouTube videos - I forget the guy's name, somebody in England - showing the different techniques. I'll try to find them and post links.
I mostly use Cleft Graft and Rind Graft techniques, but also some whip-and-tongue, splice, and wedge grafts. Which technique to use is dependent on what you have to work with, the character of the original tree as well as the nature of the new scions; also time of year is important. An old book, "The Grafter's Handbook" by R. J. Garner, remains the best text on the subject.
I have trees that were totally topworked three years ago and are now beginning to produce fruit. If done right, new growth is extremely vigorous. Basically, you collect scion wood (vigorous, preferably straight shoots of that year's growth) during the dormant season, then graft them on just before (or at) budswell in spring, when sap has started flowing. You can usually obtain scionwood from other orchardists or at scion exchanges.