These are long term projects. Yew (I suspect yours are Taxus cuspidata, the Japanese yew) can be aggressively pruned and be expected to bud back on old wood... that's why they make wonderful hedges, and I've never heard of anyone needing to graft. Ideally, this would be done while the trees are still in the ground, during the growing season, as collecting them will weaken them significantly. I have dug foundation yews and it took several years for them to recover from collection and start to grow lower on the trunk. If you're going to work with these, I'd plan on either transplanting to a nursery area in the yard or get them into an appropriately sized wooden box with good soil... collect in the spring as they break dormancy. I'd remove the trunks and branches that are absolutely not going to be part of the future styling, but leave plenty of green to fuel recovery. As the trees, gain strength, it'll be time to cut back lower on the trunk.and hopefully get growth where you need it.