Moving a magnolia of this size is a daunting task and may well fail because magnolias have a brittle root system with a limited number of feeder roots. The advice in the two responses above is worth considering, but if you're really determined to try: Prepare the new planting site first. Magnolias need a highly organic soil that drains well but holds moisture. Digging a hole and planting into unamended native soil may work for some tree species, but this magnolia is going to need all the help it can get so prepare a large - 6' or more diameter site. Magnolias do have relatively shallow root systems, so start by cutting about a foot outside the brance spread with a square spade. Gradually undercut and pry up as you work around the tree. Don't even attempt to lift the rootball. Instead, try to slide it onto a heavy duty tarp and drag it to its new site. Wide planks can be helpful in easing it out of the hole. Sometimes it possible to slice under the very center of a rootball by using a sawing action with a metal cable after you've done all you can with a spade. Keep well watered and your fingers crossed. It's a great deal of work, but I do understand the impulse to save a sizeable tree if at all possible.

Magnolias do not much like root disturbance, so moving is always a little iffy. On the plus side, magnolias have a shallow fibrous root system so you do not have to deal with a tap root or huge root ball. Spring is definitely the time to transplant in Z4. Prepare the new site first. Cut in a large circle around the tree with a square spade. Deepen the cut and gradually undercut as you go. Once the tree is loose, slide it onto a tarp or plastic sheet keeping as much soil as possible aroud the roots. Slide the tree to the new spot and plant it at the same depth it was growing. Mulch well and keep it watered all summer.

Wait until new growth is mature. I've even moved a deciduous magnolia in summer and had it do fine. The top growth had become firm beforehand. And I didn't cut the roots back.

My husband, who has past on, and I planted a magnolia tree when our children were born. I have successfully replanted it twice, one time to a new state in summertime four hours away. It has never had the luxury of being planted in perfect season or circumstances but has managed to survive each transplant. Perhaps slowing it's growth but in a few years it comes back full of life. It is dead to you if you leave it and sell your house either way. I moved the tree successfully by myself and with no equipment. If it's sentimental and you want to move it move it. If it dies, turn it into compost to place around it's replacement in loving memory. People who care more about the tree than the memory in which you planted it must not understand how those who plant with tears in their eyes drenching the soil feel about these memorial gardens. I've had brand new plants planted that have died, so planting anything anytime is always a risk. But if it's important to you move it anyway, talk to it (I did), take what original soil you can to reduce pH changes and add compost and drench it for the first few days, weeks, within reason. It should survive the move. Mine did. I was not so lucky with roses but I still wouldn't hesitate to try, if I wanted them. I didn't want to lose any of the original garden but added to it over the years. And it has given me a lot of pleasure knowing my husband's tree and memory was able to be with us. The magnolia and a few other species did the best. Good luck whatever you decide.