Now is the time.
A tree with a trunk diameter of 8" and a height of 12 ft. will require a fairly large root ball....about 36" diameter, minimum. How will you lift the tree and transport it? It's gonna be very heavy. Prune tree, and then dig out the tree, and lift the tree and root ball, including soil, onto a plastic tarp and tie up the root ball. Now, before buds start moving, is the time to heavily prune, right back to a basic shape, leaving only a few of the heavier branches to establish the basic framework of your new tree. Don't worry about smaller branches, they will regrow. Do this heavy pruning first so that the tree will be easier to lift and move. The root system is your problem. Dig out as many roots as possible. The larger the root ball the better. Keep the roots moist until you can plant the newly pruned tree back into an already prepared hole. After replanting, water it only when soil starts to dry out if there is no rain.
Unless this opportunity to move such a large tree just came about, you should have already established the root ball perimeter, last year, by spade trimming the roots...see post #9. In any event, save as many roots as possible. You'll be moving, and transplanting just the pruned back trunk with a few major branches, and a large root ball. You will probably prune away at least half of the old tree, if not more. It will regrow a whole new top in a few seasons.
You'll need a pruning saw specifically designed to cut wet, sappy, branches. Home Depot sells them. You'll also need a pointed spade/shovel, and maybe a pry bar. I would cut away all unwanted wood first, and get it out of the way. Then, start digging the root ball, even if it takes a few days, save as many roots as possible. You may have to saw through the thicker roots with the pruning saw or better yet, use a plunge saw with pruning blades. Either way, if you push the pruning saw in and out of gritty soil to cut the heavier roots, expect to ruin your saw blade. That's why you should prune the tree to size first with a sharp saw blade, then work on the root system, if using your pruning saw to cut roots.
So, basically, you'll prune, dig roots, move, and replant....done. You can seal the pruning cuts with Carpenter's Glue, or smear on some soft wax from a ring used to seal toilet bowls.
Good luck. (Move the tree now).
Frank