I am a bonsai enthusiast... so I thought I would contribute to this thread.
I would not completely rootbare a larger container tree. It is sufficent to take the tree out of the pot, gently work the bottom and sides of the rootball with water from a hose and your hands to loosen it... then using a saw cut off the bottom third of the root ball. With a small hand rake you can loosen the roots all around the sides of it and trim them with scissors, but leave it relatively in tact. Put a couple inches of drainage layer, then soil into your pot before putting the tree back into it. As with any plant make sure large pockets of air are filled in with soil.
Soil is the tricky thing... if you want a container tree to be happy it wants a certain level of inorganic substrate in the mix. It would be better to make your own soil out of composted fir bark, 1/4 inch minus crushed lava (available at most good nurseries - wash before use), and or pumice (sift the fines before use). You can do half organic and half inorganic, or a little heavier on the organic if you don't want to worry about watering so often.
Also... as a point of clarification to an earlier post, it is important to repot the tree before the buds are breaking as the extension will stimulate healing and root growth. You cannot have any root growth without foliar growth. They go hand in hand.
Other important points... acers hate to be waterlogged, and they get the best color with slightly acid soil. The composted fir bark will add a slight amount of acidity to the mix.
La belle cose prendono tempo... (Beautiful things take time...)