[The Woodman]

You could coppice - cut all growth back to just above ground level - them in the back end.
Rather than do the lot at once, try a four year plan. Do one in four this year and the same for the successive three years. Each stool will send up new shoots that will receive some protection from the neighbouring plant and so on. You'll end up with a nice varied age hedgerow.

One other way is to do the lot in one go. A bit savage and you'll lose the hedge for a couple years until the new growth gets to a height and density to replace it.