Whoa man, hold off on that. You should never do a dry transplant, not only will a dry transplant really interupt the water flow in the plants and encourage wilting, it probably the best way to severe roots and shock your plant.
You need to make sure you medium is nice and damp before you transplant, this will help bond the dirt together with itself and the roots, and it will prevent not only the roots from breaking, but it will keep the fine nutrient absorbing root hairs intact as well. Not to mention it won't make a mess of powdery dirt everywhere.
Also, why would you want to get all of that old dirt out of your roots, its perfectly fine and is supporting a healthyroot mass, removing the dirt is gonna take alot of root with it, especially if the dirt is dry, all for nothing. If someday the need arises and you really have to get dirt off your roots, like if it turns out to be plague soil, then you should get a hose and very gently wash the dirt off the roots, then repot.