I do have 12 years old, 9 meters (30 ft) tall, 4.5 meters (15 ft) wide, not-ever-pruned Leyland cypress. As you can guess, it threatens touching utility lines, and I think I have to remove it because of that danger. (Right now it doesn't touch the lines, but if I leave it this way next 10 years, it would be very difficult for an amateur gardener to remove it without interfering power lines.)
My plan is to gradually reduce it to several lateral (but curving upward) branches - over several years, so that I do not kill it by an abrupt reducing of plant mass. That way I will get several little "floating" Leylandiis in next few additional years, that would be much easier to control - or even remove altogether.
Is my plan feasible/wise/good?
EDIT: I just did the phase 1 - I removed about 25% of the tree at the bottom. these branches and leaves anyway receive much less sun than other parts, due to proximity of other plants, so I think it likely the plant will continue to live. After pruning, I noticed some small amount of sap in the area of cuts, don't know if this is a good or bad sign. I plan additional 10% removal in spring. Perhaps in a year and a half I will complete my plan.