Lilacs respond well to light pruning and can withstand losing up to one-third of the shrub mass each year without any serious threat of shock. When pruning lilacs before or after transplanting, it is always best to prune in the late spring after blooms have finished in order to minimize the loss of next season's flower buds. When pruning after transplanting, allow an interval of a few weeks for the plant to acclimate to the new surroundings and to reduce stress on the shrub.
Prune away any dead or damaged branches or twigs that did not successfully make the move. Remove dead wood back to the parent branch or down to the crown of the plant at the soil level. Thin out any branches that cross or abrade one another. Place all cuts on a slight bias 1/4-inch above a bud or leaf node.
Cut back any plant foliage, branches or twigs that appear to be suffering from disease or insect damage. Prune back to the point of healthy plant tissue to just above a leaf node to encourage branching and bushy growth of new flowering wood.
Prune the transplanted lilac to the desired height and shape by following the natural shape of the shrub and cutting branch by branch. Remove only just as much of the branch tips as needed to achieve the desired shape and size and suit the new surroundings. Again, place cuts 1/4-inch above a leaf axil or bud and on the bias, so water will roll off of the wound surface.