if you're not messing with the roots it'll be fine. just take it out of the pot, add soil to the barrel, place the tree in and fill in the sides with soil.

if you are going to be messing with the rootball, any kind of root pruning, etc, like what the bonsai growers do, then you need to wait till its dormant.

Pretty much what HT said ....There's a difference between repotting and potting up. Repotting includes root pruning and a fresh change of soil after removing all or most of the old soil. Potting up is simply transplanting the tree in a larger pot with a slightly larger soil volume. Potting up ensures continuation of the limitations inherent in congested roots, repotting relieves the tree of that condition. Unless you encounter some sort of emergency, there is little reason to repot a deciduous tree while it's in leaf - the potential problems are many. Pot up any time, but it's best to do it when the tree is growing well or about to put on the spring flush.

Note that the growth "spurt" you usually see after potting up isn't a spurt at all. It's an indication that root congestion is and has been limiting your plant's growth and vitality. The 'spurt' is simply a result of the tree filling the extra space with fine roots, which temporarily allows it to grow somewhat closer to its genetic potential.

Al