DANK,
I would do the following:
Cut your current plant off at the base of soil, trying to remove most of the "trunk" from the rootball. Leave the rootball undisturbed in your container. Next mix up a mulch to lay down right on top of your soil. A good mulch would consist of ewc (home made is best if you have a worm bin) along with a good dynamic accumulator such as comfrey, nettle, yarrow, etc. If you have something like comfrey just dry the leaves out, and crumble them up a bit. I would also add some aeration material to that so that your container doesn't get too muddy. Rice hulls are a good option for this. You should also re-amend your soil with some organic granules such as kelp meal, alfalfa, and/or an all purpose granule like one of Espomas "Tone" products. Aproximately half the amount of organic inputs that you originally used to amend that amount of soil with. Once your mulch is prepared, broadcast some clover seeds in the mulch mix, and then add a blanket of bark chips on the very top (optional, but recommended). This will keep the soil moist and alive beneath it, and reduce the amount of times you have to wet down the container. Using an ACT to wet it down with is a good idea, imo.
At this point your container is ready to sit for the requisite amount of time. I would leave the container in your flower room, but off to the side or at the perimeter of your current mj plants that are under your light. This will give the clover some indirect light, but not take up any room directly under your light. Letting it sit like this will allow the microbes in the soil to begin breaking down the rootball. I would leave it be for a period of 30-60 days. At that point you will have a nice layer of clover that you can "chop and drop" .... meaning that you cut the clover down, but leave it on the surface of the soil. This clover will act as another layer of mulch when you add your next mj plant to the container. There will be a ton of micro-life near the surface due to the ewc/bio-acumulator/organic ammendments/mulch that you laid down. Now just dig down a bit and create a pocket/hole for your mj plant to be placed in, and situate the mulch back around the base of your new plant. Done deal! Water and proceed as usual.