My guess would have been that you already have them growing. At least, here in Northern germany all my junipers are growing fulll swing for a few weeks already, and I finished repotting.
I guess it depends how dry it gets. You can transplant junipers for a few months in spring, assuming you gets lots of humidity in the air. Dry winds are killers for recently collected junipers.
What you could consider doing is cutting the main roots now on maybe 1/2 of the rootball. Take a spade and push it in, maybe 30cm (1ft) fro the trunk. Then rotate a quarter turn around the trunk, and do it again. Do this untill you have gone around. You would now have cut half of the roots, in 4 sections of the rootball. When you harvest next year you should have a more condensed rootball.
Or you can just go for it. Transplant (Try to not clean out the rootbal, but chuck it in a pot). Then keep the roots relatively dry (Not dusty but certainly not soaking) and mist the foliage as often as you can (Keeping the water from extensively dripping into the pot. Give it some morning and evening sun (Assuming california midday sun to be brutal already) and protect from wind.