I agree with RpR if you are going to dig it up and move it! Why not cut the horizontal branches to the soil? When we move into our house we had lilac that
were over grown and horizontal too. Knowing if you cut it now, you will not have
blooms next spring. Ours came back like gang busters!! Every so often I tend to
cut it back to the soil due to I don't want it taller than my 6 foot fence.
