In this video Stephen Hayes of Fruitwise Heritage Apples describes how to graft over an apple tree.
Top working or grafting over an apple tree which is sound, but not the right variety for example. This example is a Crimson King that has been grafted over to other varieties. A pencil of the chosen variety is cut in February and kept cool until around April when the tree is getting going. Saddle grafts and cleft graft techniques. Probably best not to let the graft fruit the first year, let the joint strengthen first. Will fruit heavily in only three years from being top grafted over.