I recently purchased a 5 year old chinese elm from the Brussels nursery on amazon. It seems to be doing fine and has exploded with new growth this spring/summer. That being said, I've noticed that the soil it arrived in doesn't drain as quickly as I'd like as it can stay damp for up to two days depending on the weather. Living in Eastern North Carolina, it can get extremely humid and rain often so I assume a better draining mix would be optimal. My question is whether or not it would hurt anything if I were to simply remove the tree and replace the substrate without doing any root pruning. The substrate I was planning on replacing it with is a pre-made professional mix made of equal parts japanese akadama, japanese pumice, and black lava. We're expecting a lot of rain in the next couple of weeks so I'd like to get this done ASAP.

If you know what you are doing, this is an option. If you do not know what you are doing, chances are high you kill a tree with this action.
As you are asking, I assume you do not know what you are doing, and as such will likely kill the tree.

The risk of getting trouble through the soil would be lower than the risk of killing by improper repotting in the wrong season. I would recomment leaving it, and just be carefull watering.

hoody wrote:

I recently purchased a 5 year old chinese elm from the Brussels nursery on amazon. It seems to be doing fine and has exploded with new growth this spring/summer. That being said, I've noticed that the soil it arrived in doesn't drain as quickly as I'd like as it can stay damp for up to two days depending on the weather. Living in Eastern North Carolina, it can get extremely humid and rain often so I assume a better draining mix would be optimal. My question is whether or not it would hurt anything if I were to simply remove the tree and replace the substrate without doing any root pruning. The substrate I was planning on replacing it with is a pre-made professional mix made of equal parts japanese akadama, japanese pumice, and black lava. We're expecting a lot of rain in the next couple of weeks so I'd like to get this done ASAP.

I've done this. All you do is carefully remove the tree without touching the root ball. You can actually get some soil off the root ball if you are careful, but I really don't recommend doing this. Anyway, remove the tree off the pot and leave the root ball intact. If the soil is a bit dry this will work better as some of the soil on the lower part of the root ball will naturally fall off. Then you put it in a new pot with 100% inorganic soil, like small pumice. This will greatly improve drainage, and some of the bad soil in the root ball will slowly mix with the pumice.

I've done this in late summer and it works just fine, it is not that hard to do. You will see water coming out the drains, and a much better drainage. Next year, you re-pot it. You can also leave it as leatherback suggests. Where I live, winters are extremely cold, and all my trees live outside year round, soil with very poor drainage calls for rootball freeze damage.

eangola wrote:

remove the tree off the pot and leave the root ball intact. If the soil is a bit dry this will work better as some of the soil on the lower part of the root ball will naturally fall off. Then you put it in a new pot with 100% inorganic soil, like small pumice. This will greatly improve drainage, and some of the bad soil in the root ball will slowly mix with the pumice.

The big risk here is that the rootball dries out, and does not absorb water anymore. Water runs off along the side of the rootball, and the main roots start to die-off, leaving no growing tips in the core and then plant depending on a very smal proportion of the rootball. .

hoody wrote:

We're expecting a lot of rain in the next couple of weeks so I'd like to get this done ASAP.

Why not just protect it from the rain in this situation.

Personally, if the tree is healthy and doesn't seem to be suffering from the soil, leave it alone. Monitor the amount of water it gets and put in the extra effort until next spring.

I've repotted a plant at the wrong time of year to "prevent issues", and it only weakens the plant, making it more susceptible to the very things you are trying to prevent.

You can go with eangola's suggestion. I've done this before, but leatherback is right. Don't over pot, and make sure that the tree is getting quality waterings, to ensure that the tree gets the moisture it needs. Your waterings will have to go off of when the root ball is dry, not the pumice, which can be difficult depending on how you pot the tree. If it's a small tree, you may be better off just moving it out of the rain, and sheltering it in the winter.