This is assuming you have Lavandula angustifolia or Lavandula x intermedia. The best time to transplant lavender is when it is dormant or just breaking dormancy. Keep lavender in check by pruning 1/3 to 1/2 of the plant every year. For us the best time is now.
When tranplanting, be sure to cut the top to approx. the same size as the root ball. Remember, you are pruning the roots when you dig up the plant so you need to prune the top too.
If the plant is very old it may not transplant very successfully. If you are particularly fond of this plant, root a few cuttings just in case the plant dies. I have moved a few lavender successfully but it was not easy, I needed a small tractor w/ a loader to help me get the largest root ball possible.