I would obtain a slightly larger pot and only pot up for now, saving the root work until late Mar/early Apr. You can then reduce the planting and probably put it in even an 8" pot if you like, but certainly a 10" pot would be no problem. Also, if the plant IS healthy, you should be able to just slip it back into another 10 incher, as is. Al

I grow several dozen vegetable plants in large containers, and annually purchasing new soil would be prohibitively expensive. Most commercial potting soil is some combination of perelite, vermiculite, sand, and one or more organic sources, peat moss or "forest products" or some combination. The organic stuff decomposes over time, resulting in a a higher concentration of stuff that won't break down, so one needs to add organic stuff, like compost, to "rejuvenate" the potting soil. I usually let it get really dry, then dump all the pots out together on a concrete slab, smash up the root balls with a shovel and shake out the big parts, add a few wheelbarrows of compost, mix well, and refill the pots. 15 years and counting.

Bamboo are not like other plants in that they don't benefit from having rootbound roots sliced up; in fact it can set them back. If you've got some loose roots that you can position, I guess that wouldn't hurt, but be careful not to bend them too much. You have two basic options available: 1) unpot the plant and put it in the ground without messing around with the root ball; 2) if you've got that many roots and more than one culm, you might be able to divide it in two with a root saw and have two plants. In general, don't worry about a rootbound bamboo. It will be happy to get out of the pot and will do well all by itself.

I would Repot!