Usually when you buy plants from the store, the soil type might not be suitable to them. Is it safe to change the soil completely? I mean to remove all the soil around most of the roots, then put it into a pot with new soil. I can think of 2 disadvantages:
Would it help if I only changed the soil away from the roots? Are there certain plants more tolerant to such changes and others that aren't?
Generally speaking, plants bought from a store or nursery are grown in compost (potting soil) that suits their needs, otherwise they wouldn't grow well and wouldn't sell. Unless the plant is unhealthy or pot-bound (the pot is so full of roots that a mass of them encircles the soil-ball), there is no need to disturb it, in fact changing the compost is likely to do more harm than good.
If the plant is pot-bound (if so, you can often see roots growing through the drainage holes, or water runs straight through the compost, without really wetting it), it will have to be moved into a slightly larger pot. Water the plant very thoroughly, place the base of the plant between your fingers, turn the pot upside down or on its side and give it a sharp tap. Gently remove the plant and tease apart the roots that are encircling the root-ball, but without disturbing its overall structure; the aim is to keep the root-ball intact. You can then transfer the plant to its new pot, using a compost that contains some of the ingredients in the old one, e.g. if the original compost is peat-based, and the new one you are using is soil-based, add a little peat, so as to ease the transition and encourage the plant to grow into the new medium. Just place enough of the compost at the bottom of the new pot and fill the gap around root-ball, again without disturbing it, and then water thoroughly.
If the soils types are completely different, even though washing out all the soil may damage the roots, sometimes that is necessary. We have soil that is almost sand, and if the plants are left in the original rich mulch and planted in that soil, the sand sucks all the moisture out of the mulch and leaves the plant stuck in a hard, dry medium. If the mulch isn't washed off the roots, many plants can't survive.
So while usually it's much better to leave the plants in the soil they came from, in a few cases that is not the best option.
When we do change out the soil, we dig a big hole, and keep the plants in water and mud while rinsing. There is a LOT of water used in the process, so the roots are gently rinsed and never get dry.
Remember that "changing the soil", or even a simple transplant is going to damage the roots. You will want to minimize root damage as much as possible, so you need to balance "changing the soil" with damage. I would suggest that in most cases, you probably only want to break away the edges of the soil and not the "central core". The latter could cause a lot of damage and probably won't change things too much.
Note that if the store has used the wrong kind of soil then it probably isn't too incorrect - so it won't be too critical if a lot of the remaining soil remains in the middle. If it is really wrong (eg. carnivorous plants in a very sandy soil, or cacti in a water-logged peat moss) then you should find a new store.
