The question of how hard you can cut back this hedge is a tricky one. It sounds like you will be reducing these hedges by several feet which is a large amount to remove and the plants will find it stressful. 

If your shrubs are in very good health, this severe reduction is possible, but it may take some time (a few years) before the hedge tops look healthy and green again. Yew (Taxus) is excellent at regrowing from older, woody parts of the branching structure but it takes time and is not entirely without risk for uniform results.

Early spring or late winter, just before new growth is initiated, is the time to cut. Use sterile cutting tools to protect the wounds from infection. Check if the soil under these old plants needs some fresh compost added to help with nutrition and soil texture. That and an eye on watering during dry spells as they regrow will help to support the plants.

Courtesy of NYBG Plant Information Service