You can repot in better soil mix once the trees are fully dormant......no need to wait until spring.

It depends on what the potting soil is like. Whether it actually needs to be enhanced or not. There are multiple different formulations on the market with varying characteristics, so we can't tell if the one you used would best be doctored.

With any sort of containerized tree you always want a bark-based potting soil. And since few, if any, commerical potting soils are bark-based, it is pretty much a given that the potting mix will need amending.

I concur with Gardengal, I would not use anyting that contains more than 10-15% peat based potting mix. I have my JMs in Gritty Mix, which contains no potting mix at all and only 1/3 un-composted Pine Bark, and they are doing exceptionally well.

Just cut back your watering over the winter, and repot early next season.