I think the best advice was said in the beginning of this page. Go ahead and buy new soil for your cannabis. It's the best way to ensure the results YOU want and the money you save by buying your own should make it perfectly affordable. If you can't afford to invest money in your soil, I dont know how you picked up a marijuana habit to begin with. lol
Well, again, in this very specific case, if all the original poster wants to do is replace one young clone in new soil with another clone, it seems a waste to swap soil just for that.
Generalizing more about what to do at the END of your grow, swapping or not swapping soil isn't solely a question of money, but also efficiency. If you're really into "sustainable" organic gardening, then you simply might not want to start again from scratch each time you grow, and if you're already making compost, you may not need to. Not everyone has an outdoor garden they can dump their old soil into for re-use (eg some people do grow in urban areas, for example, nor does everyone with an outdoor garden want to do that.
Convenience is a factor too. Being able to resuse soil can save you a trip to the store (which may not be that convenient or easy to get to), and/or to the dump. Not everyone is shall we say, growing "under license" or in complete seclusion. Some people grow in apts or in other situations with other people around. If so, bringing large quantities of soil into and out of the place every few months may not be exactly "inconspicuous".
In terms of how/when to recycle, that's going to depend largely on what kind of growing you're doing and how. Growing organically is different than inorganically, and indoors is different than outdoors. Yes, every plant has its own type of soil that works well for it, but ultimately cannabis is just another flowering plant that thrives under a wide variety of conditions all over the planet. It doesn't have special needs completely distinct from every other plant out there. Again generalizing, if your soil is good enough to grow common household vegetables, it "ought" to be good enough for cannabis as well.
If you HAVE to reuse your soil for weed, I remember a while back when I first started growing, they mentioned you can bake your soil in the oven at 300 degrees for an hour or so. Sift through it. Then ammend with nutrients.
Pasteurizing/sterilizing your soil as you describe is especially good practice if you're bringing outdoor soil indoors. Outdoors soil is typically full of insects, other live organisms, and can be full of seeds or spores. You don't want that stuff contaminating your indoor garden if you can help it. Note that many of the commercially available garden/potting soil mixes are NOT sterile in the bag either!
On the other hand, if you've actually tried baking soil in your oven for an hour, you might understand why some people don't like to do it! (Hint, the smell can give new meaning to the term "mud pie"!).
If you start clean, grow solely indoors, and using good growroom hygiene (including quarantine of plants brought in from outdoors or outside sources), then you shouldn't "have" to worry about pests and/or most diseases. If they're not in your room when you start, they should stay out *unless* you bring them in or otherwise let them get in. If they're already in there, or can readily get in, starting with new soil isn't going to help.
Lastly, if you're really going to worry about whether your used soil is good enough or not for your grow, then by all means, change it.
Even if new soil isn't really going to improve the quality of your grow, it may be worth it just to improve the quality of your sleep!