Answer: This is a huge lilac to try to transplant, and it will have a proportionately huge root system. It will be very difficult to dig and extremely heavy to move, so you may want to consider planting a new lilac in the new location rather than trying to do this.
To transplant you would cut it back to the ground, dig it up with as much of the rootball intact as possible (it will start further out than the branch tips and go down very deep), and replant immediately at the same depth as it grew before. Do not let the roots dry out while it is in transit. After planting, water thoroughly to settle the soil and mulch. Water as needed to keep the soil evenly moist (not sopping wet or saturated) just as you would for any new plant.
The best time of year to do this would be late fall or very early spring as soon as the soil can be worked. It is already a bit late for this spring.
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