You are correct being worried about moving a 12" caliper tree...bare root? Nope. Only way to do that with a 50% chance of success is to begin at least one year before transplanting by digging a 1 foot trench around the major portion of the root ball with a 4' radius. In no way could a homeowner move an 8' diameter root ball. Smaller younger trees perhaps with a 2' root ball that could be moved with nothing than a tarp or a 'ball' cart and a dude or dudette or two for help is a big production. In the meantime, while you figure out the logistics and if you are super sure you want to try moving this tree you need to do the following:
Dig a trench around the base of your tree at least 4' radius. 1' deep. Cut roots as you go. Pack fresh straw into the trench. Allow your tree to get used to using the root ball you've left. Even during the winter, those roots will be growing new roots within the root ball. Obviously it is a good thing to start in the fall...allow your tree to acclimate to fewer roots during the season where less water is necessary. Ideally you would wait for transplanting until the second spring where you'd dig up the tree and move it to its new home You might be able to try to move it the first spring but your chances of success are greatly diminished. Greatly.
The second fall you would make the root ball even smaller by digging a trench at a 2' radius. You would still need a professional tree mover to pluck up the entire root ball and then put it in its new home right away.
8' wide root ball is a bit much for a homeowner to handle! A 4' is possible with a professional tree moving company. Fill the new trench before the second winter with fresh new straw. Use the old straw in your compost pile. Soak these straw filled trenches before the ground starts to freeze. No fertilizer unless you are doing the fall to second spring time. Then your formula should be equal N and P and K. 10-10-10? Tree stake? Apply in the late spring. The next spring you will be moving your tree. I doubt you'll be able to move a 4' wide root ball by your self or even with help. Need a big machine or at least a back hoe with 4 or 5 stout dudes to transplant a 4' root ball 12" caliper tree (this is 12" on a ruler that you hold up to your tree that shows the diameter of your tree, correct)? 4' root ball might be a big stretch. Professionals will take care to tell you what you can or can not expect. They don't have a service without success.
By trenching and preparing this way you should have at least a 75% chance of survival. Otherwise by ripping your tree out and transplanting it bare root I can guarantee you will lose your tree and all the money you spent to move it with this company.
You should also get some Mycorrhizae Fungal spores at your favorite nursery. Make sure you apply to the first trench you dig, lightly. I would also look into a tree moving service. They have a big machine made to pluck out trees and shrubs by their root ball. The larger the root ball the more chance of success.
When you prepare the spot that will be you tree's new home (hopefully not too far away) do NOT dig any deeper than the root ball you will be moving to install in that spot. Do not disturb the soil that root ball will be sitting upon. No amendments whatsoever. If you are on a slope you'll be making a tree well on one side and inserting a 3 or 4 inch PVC pipe at least 3' long that has been drilled full of holes for about 2' of its length. The undrilled portion will be above the ground. I would do at least 3 pipes around the edge of the root ball after transplanting. You put your hose end in the pipe to water your tree. Keep filling until the water doesn't flow out of the pipe very quickly.
For the first spring and summer that tree needs water! The pipe will allow the subsoil to hold moisture where the roots are able to get at the water. Check the moisture level by pulling up the pipe or drilling a larger hole about 6" below the surface in the pipe where you can feel the soil through the hole. (you can get a hand into a 4" pipe)? Once transplanted that tree will need a good 2 months where you need to keep that soil moist not wet but moist 24/7.
Sprinkle the mycorrhizae into the new hole when transplanting. Minimal fertilizer...if you used a stake the last summer that should be enough to keep it going until established. Then use a tree stake fertilizer once every other year.
Make dang sure that the bark of the tree is out of the soil and not ever to be compromised with mulch or bark or rock...or weeds or compost. There is a line between the root system and the bark. That is where the soil needs to cover and not a quarter inch above the roots should the soil, mulch, compost, rock...whatever...touch. This is why you do not disturb the subsoil. If you did disturb the sub soil then your tree will sink lower and any soil, or mulch that covers the bark will cause moisture and bacteria the ability to start decomposing your live tree and fragile cambium...the vascular system just beneath the bark. That will eventually girdle your tree and cause it to die.
Transplanting a 12" caliper tree is not an easy thing to do, at all. If you want success you will have to prepare ASAP and give that tree at least a year if not 1 1/2 years to acclimate to a new root ball size.
This also be a good time to prune your tree reducing its 'overhead' so to speak. You will only be doing thinning not heading. This reduces vegetative growth that needs carbohydrates (made via photosynthesis), reducing the need for water and reducing the stress of this transplanting process.
When you decide what you can afford to do or want to do it would be fun to lead you through a proper pruning to be done ASAP possibly in a few sessions not just one before your tree needs to deal with the big move.
No way are trees of this caliper EVER bare root transplanted. Kiss that tree good bye.
I have to say to replace mature trees is costly and time consuming. A simple Japanese Maple 12'X12' removed from a residence as I have recounted, potted and made for sale will cost you 10,000 bucks. Easy for people who are richer than necessary...where is it that you live, where are you moving this tree to (same site, same soil, same slope as before)?
Do contact some of these 'Big Tree' services. This is the process they demand or they will come and rip out your tree, shrub...rhododendron...and chip it. If this process is not done, that tree is toast. I worked with 'Big Trees' in Snohomish or Arlington north of Seattle, Washington. Call them and talk to them about their services, don't let them know you don't live in the area...tell us in more detail what you are dealing with and we shall be able to help more closely.
Do not do bare root transplanting of any kind for your tree...disaster.
I'm not bragging but I am a Landscape Architect. Licensed and 4 decades of experience of hands on gardening, maintenance after the installation of the design. Turning clients into Gardeners. At least educating them enough so they can check their service's credentials and/or educate their gardener's service into doing the proper techniques for landscape health.