That all depends.

If you plan on changing the mix your tree is already in into an entirely different mix then a bare root would be best. Carefully washing all the the soil of the root mass. If you need help with that just ask)

If you plan on using about the same kind of mix, then you could just tease the roots a bit to loosen up the mix already in the root ball so the that roots will seek out the fresh mix, and then replant into a fresher mix.

Mike