Moving deciduous trees, like maples, is best done when they are snoozing in winter.
Before attempting to do anything with your tree, the first thing to consider is timing. Winter is the best time to transplant, re-pot or heavily prune a deciduous tree, because it’s dormant and therefore won’t suffer the stress this sort of work can cause.
Start by removing the hessian and dismantling the broken pot. For larger sections, carefully use a lump hammer to break up the cement, then remove. Now the root ball is exposed, use a sterile and sharp pair of secateurs, loppers or a saw, to remove damaged or problematic roots on the sides of the root ball. Ensure the cuts are straight and clean because torn, blunt cuts encourage infection. Don’t worry about hurting the tree – it really is fast asleep and won’t even know you are clipping anything away.
There could be problematic roots on the underside of the root ball that aren’t visible, but may need removing. Using heavy-duty straps and the help of another person, position the straps under the root ball and tip the tree to one side to sever any roots, which then need to be trimmed.
Position a new, bigger pot in your desired location. Part-fill with potting mix, lift the tree (again, using straps) and place inside the new pot. Backfill with potting mix and water well using a seaweed solution. If necessary, also prune any damaged or unwanted branches. Or alternatively, replant your maple into a new spot in the garden. It will love its spacious new home!