Sooner the better. I transplanted an Acer Palmatum 'Nuresagi' this past week after projecting how large it would grow. It got reduced last winter to a couple of branches, but was nonetheless pretty healthy. However, I didn't know in early Spring since it looked pretty pitiful then. I planted an AP 'Osakazuki' far too close to it. Late Spring the new growth was prolific--far more than expected.

It started out last year as a three gallon, but I transplanted it about three feet further so the soil is similar. I dug around the roots and underneath and was able to move it with most everything intact. I've watered it everyday by giving it a good soaking and it looks ok so far. Yours is even smaller so I would think it would be easier to move. Just treat it like a new planting because by moving it it will have to adapt to the new location no different from a new plant.