You MUST remove all of the old soil. Rinsing in a bucket of room temperature works good. I like to fill a 5g bucket, and let it sit for a day, then soak the rootball for a while to loosen the old dirt. Gently work it free in the water until all of it is off of the roots. Then, hold it in the new pot in it's approximate location, and fill in with gritty mix, making sure to fill in around the roots. It helps to use a dowel, chopstick, etc. to push the mix around. I also tap the pot on the sides to settle the gritty mix down around the roots. Make sure not to leave any soilless pockets, paying special attention to the rootmass directly under the trunk.

Joe

Precisely what Joe said.

Josh