moving lilac trees

It would be better to leave it until the plants are dormant next winter. However if you have to do it soon, prune them down and then dig out with as much rootball as you can. Keep them well watered all summer and keep your fingers crossed!

As the trees are so tall, it's a fair chance they will have an extensive root system.
If you can afford the time, about one month before you intend to move the trees, with a sharp edged spade cut a deep groove in the ground about 2-3 ft away from the tree.This will sever distant roots and give the tree chance to recover. Keep the cut clear until you are ready to move the trees.
When you eventually move the trees, make sure you have the hole you intend to plant them in, already dug, well watered and with a good layer of compost in the bottom. If the trees have to remain unplanted for any length of time, wrap the rootball in wet burlap or sacking and keep it cool. The ideal time to do this is January to March.