I would add 5 or 6 bags of compost or manure in with the clay 4 or 5 feet around there and down. Then as you said get rid of the potting soil and spread out the roots. Prune the roots if necessary. Top dress the top with 6 inches of wood chips about 5 or 6 feet around. Keeping it moist will be critical the first year. See what i did with smaller bushes
The results