Answer: The best way to regain control would be to gradually prune out the older thicker stems over the period of several years. Do the main pruning in late winter before they start to grow for the season. Reach into the center of the plant and selectively trim one branch at a time. The first year trim back about a third of the long branches, then in year two trim back half the original big old branches that remain, and in the third year trim the the rest. Meanwhile, trim the new growth often from spring to mid summer to encourage it to branch and stay compact.
With yews, you can also try a very hard renewal pruning. To do that, wait until late winter and then trim the entire plant off extremely short. Trim often as it regrows to encourage branching and dense growth. This treatment is a bit risky in that it can sometimes kill the shrub. Also, it takes a year or so for the plant to look halfway presentable again after this treatment.
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