'FAQ:2004:5 - Lily Bulb' (Info)
New patio or city road/water/power line work endangering your lily garden in bloom? If you are very careful, by digging up the entire "clump" with a good root ball of soil, you should be able to move them without any damage.
Dig a hole slightly bigger than what you need, fill with water and some loose soil to create "mud". Place the entire clump into the hole, filling in and gently tamping more loose soil down around the root ball. Use just enough water to firm the soil and withhold water until the soil has started to dry out, typically 7 to 10 days. Do not over water!
This method should only be tried under emergency conditions, the best time to transplant in the fall after the foliage have matured, so don't try to save time and do any dividing right now. If you do not expose the bulb and roots to air, they will never know they are in another location. The biggest mistake is to give the bulbs extra water to cover "transplant shock"; if the lower leaves turn yellow and fall off, they received more moisture than they could handle, and you need to stop watering until the soil is dry at least two inches below the surface.
In spring, if the bulbs are just beginning to break though the soil surface and need to be move, dig them very carefully, so you do not accidentally break a sprout. If the sprout is broken, it will not grow another one the same season and you will have no bloom from that bulb.