i wouldn't worry about it too much, i once transplanted 4 5-foot tall plants in icecream buckets, if your plants have been well cared for the root ball should be fairly small. if it gets really dry where you are they may have made longer roots but cutting them isn't going to kill the plant at that size, as long as the majority are still in the ball, the main tips i would give you are, have your bucket or tarp ready when you dig, dig as big a ball around the plant as you can lift, and quickly move it as fast as you can without bumping it too much, and watter thourally after, your plant will probably droop pretty bad but will recover, you will minimize death if you do this in the evening, so they have all night to recover before the sun bakes them again.
as for different soil, unless the ph is way off, like if it's pure peat moss or came from under a pine tree, your plant will probably not be affected at all. just use good soil and your plant will be happy