Moving Lavender Plants

Your Q&A pages are invaluable, and I thank you for them.
I have a question I couldn't seem to find an answer for. I will be moving in 6-8 months and hope to take with me my 200-300 lavender plants of all ages.
I've heard of root pruning trees to slow their growth now, and wondered if I should do this first rather than hauling up the whole root ball and trying to cram it into a huge pot or box; I know this would damage the plant, but whateaver course I take will happen in November or December when I hope the plants are reasonably dormant.

Nurseries prune the roots of trees and shrubs in order to make these plants easier to transplant. Pruning the roots encourages plants to generate a denser, more compact root system. Without long roots penetrating wide and deep, it is much easier to dig these plants up, with less transplant shock.
In theory, pruning the roots of lavender will have much the same effect. However, in our experience, lavender transplants easily, and there is little need for root pruning. Rather than trying to preserve the crowns when transplanting older lavender bushes, we try to divide the crowns. The crowns of older plants are so dense with branches that fungus diseases can easily attack, eventually killing the plants.
We are tempted to transplant after flowering. By now (September) your plants should be well past flowering and in seed, so this is a good time for digging and dividing. To facilitate the move, you could plant into large pots that are set down into the ground. When it comes time to move these plants will be easy to pull up and transport.

I've also been reading about Provence lavender, and think it might be easier to chuck out all the Munstead and Hidcote I have and replace them with this. Would Provence do all right here on the Saanich Peninsula?

Provence is perfectly hardy. If Munstead and Hidcote are doing well then Provence is likely to do as well. All lavenders need excellent drainage, slightly alkaline soil, and full sun in order to thrive.
Why would you dispose of the Munstead and Hidcote plants? These two are of the English type, Lavandula angustifolia, the hardiest and, we think, the finest scented type of lavender available. Provence is a very good variety, and is larger, more fragrant, and more disease-resistant, but the English types such as Munstead and Hidcote have a finer, purer scent. Provence is a hybrid of L. angustifolia ancestry, but its subtle camphorous or resinous overtone comes courtesy of its L. latifolia parentage. While the Provence oil is definitely high quality, the plants main virtue is really its size and yields compared to the true English lavenders.