Repot ball trees when needed, not by a calendar. Several reasons to repot: The tree has grown too bit so looks funny in the pot; roots have grown and completely filled the pot (most common - happens every 1-6 years with younger trees. Test by pushing down on the soil. If it is rock hard repot, if still spongy leave it); soil mix has decayed into sludge and needs replacing; You want the tree to grow faster (repot into a larger pot or into the garden for a couple of years)
The current pot looks fine for size unless you want it to grow quicker. Before deciding whether to repot you really need to decide what you want from this tree.
Asking about soil will reignite soil wars III. Everyone has a favorite soil mix but they are all different and everyone firmly believes that theirs is the best and only one to use. Find out what other experienced bonsai growers in your area use and try that for a start.
I repot juniper down here in mid spring but many growers now repot after mid summer. They don't like real hard root cutback but I would not expect a newbie to be game to cut off too much. Up to 1/2 should be OK if you do it. Juniper also have a reputation for not liking too much pruning or shaping at the same time as repotting so most growers limit juniper work to one major process each year. Down here procumbens seems to be much more forgiving an I can style and repot without adverse outcome.