In the Northeast yuccas should be transplanted in late spring or early summer.

With a sharp spade dig around the plant at a distance of about a foot out from the plant  Then dig under the plant to a depth of about 9 - 12 inches. Work the spade back and forth to cut the roots.  Try to leave as many roots intact as possible. It may be wise to wear gloves and goggles when doing this to avoid problems with the sharp thorns that yuccas carry.

Then, using the spade, lift the whole plant on a tarp to move it to a new spot. The plant can be divided at this stage using a sharp knife or a spade if the plant is very large. Some gardeners recommend letting the roots dry out (in a shady location) for a few days before replanting.

Make the new hole about twice the width of the plant but the same depth. Fill in the hole with soil and tamp down just enough to fill any air spaces. Make sure that the yucca is not planted too deeply and the root crown is at soil level. Orient the plant in the same north-south direction as it faced originally. Water the transplant and again every week until it is well establish Do not overwater as there is a danger of root rot.

Courtesy of NYBG Plant Information