Since you edited your question, I would like to expand my answer.
What you plan is to let raw milk go sour, then add a yogurt culture. You ask what will happen. The answer is: it depends on whatever bacteria were in the raw milk in the first place. And you have no control over that. Here are all the possible outcomes if you start with milk not contaminated with any harmful pathogens:
- The milk its on its way to become a tasty and harmless yogurt even before you culture it. This will happen if the initial bacteria were harmless yogurt-producing lactobacilii, and they dominated all other bacteria species in the milk.
- The milk is on its way to become something tasty and harmless, but different from yogurt, before you culture it (for example the cottage cheese you mentioned). You add the culture, the yogurt bacteria dominate the non-yogurt bacteria, and you end up with yogurt.
- The milk is on its way to become something tasty and harmless, but different from yogurt, before you culture it (for example the cottage cheese you mentioned). You add the culture, the non-yogurt bacteria dominate the yogurt bacteria, and you end up with something different from yogurt.
- The milk is on its way to become something yucky and harmless (because it happens to contain the wrong lactobacilii). You add the culture, the yogurt bacteria dominate the non-yogurt bacteria, and you end up with tasty yogurt.
- The milk is on its way to become something yucky and harmless (because it happens to contain the wrong lactobacilii). You add the culture, the yucky bacteria win, and you end up with something yucky which is not yogurt.
I doubt that 4 can ever happen, because once you have produced the bad taste, it stays there even after the bacteria responsible for it die out. Your position that raw milk goes sour, while pasteurized milk goes bad can be then understood as stating that case 5 only happens with pasteurized milk, and raw milk will produce result 2 or 3, and you are asking us if 2 or 3 will happen.
Even if we assume that 5 really can't happen with raw milk, you still can't predict whether 1, 2 or 3 will happen before you have the end result. It depends on which kind of bacteria will invade your milk, which kind of yogurt-making culture you will add (there are lots of them), and what conditions you will keep the milk in before and after the culturing. If you try it, you are practically gambling. You can end up with yogurt, with cottage cheese, with kefir, or with some strange, inedible combination of all of these. That said, I suspect that 1 will be much rarer than 2 or 3, just because the subset of lactobacilii which produce yogurt is somewhat small. It still can happen - it happened in my own kitchen with pasteurized milk forgotten on the counter for a weekend. When I came back, it had the texture, viscosity and smell of yogurt, not cottage cheese or something else. (I didn't taste it, so there is a small probability that it was case 5 after all).
You can try to make your chances for getting yogurt higher by choosing a more aggressive and rapid growing yogurt culture, and using big amounts of yogurt culture as starter. This will make it easier for the yogurt culture to overwhelm the other culture, because it will grow at an extremely high rate. As a downside, the rapid fermentation process will result in a sour, pungent yogurt with a high ratio of acetic acid and quite probably some ammonia.
The reason why Sobachatina and I reacted so strongly against your idea is that there are five other possible outcomes. For each of the five cases above, if you start with a milk which is contaminated with a pathogen, you end up with the same result but it is no longer harmless, it is very dangerous - let's call them cases 1b through 5b. And there is an ugly detail: Milk (or yogurt, or cottage cheese) which is contaminated with something nasty does not taste, look or smell different. So, whatever you end up with, you have no way to tell (short of a lab test) if you ended up with 3a (what you wanted) or 3b (a yogurt which will make you sick). The fact that you bought raw milk from (presumably) a well-handled, healthy animal does not make the risk of contamination low enough. And while you can find reputable sources which will tell you that raw milk doesn't "go bad", what they mean is that case 5 doesn't happen with raw milk, not that raw milk is never contaminated with pathogens.
I have no means to stop you from conducting your experiment, but my advice for you and anybody else who might read this question is to not do it. As evidence for this opinion, I will leave my old answer below. It is based on peer-reviewed articles and government-approved guidelines, not hearsay or unattributed web sites. For anybody still unconvinced, I recommend reading the articles themselves.
You don't need people with "experience" of raw milk. In food safety, experience doesn't help any; counting pathogens does.
Common sense alone should be enough to notice that spontaneously fermenting raw milk can't be a good idea. Look at how hard it is to obtain raw milk - wherever this is possible at all, it is very tightly regulated. The reason is that cow stalls are dirty places, and milk is a perfect growth medium. If it was safe to leave raw milk around and eat it after it has fermented, nobody would create such tight regulations to prevent this from happening. Making yogurt is a special case - if you have a big colony of harmless bacteria, they can outcompete pathogens quickly enough (but still it is preferable to make yogurt with pasteurized milk, just in case). To give you an example: in Germany, raw milk can be sold up to 24 hours after milking, and has an expiry date of 96 hours after milking (and has to be kept under 4°C in the meantime). The raw milk must be labeled as such, there is a warning that it should be cooked before consummation, at-risk populations (pregnant, elderly, infants) shouldn't have it even cooked, and it is forbidden to use raw milk in commercial kitchens.
But as I said, you shouldn't rely on common sense where it comes to food safety. So here the hard data. I went to pubmed and searched for "bacteria in raw milk". This is a complete list of the bacteria mentioned on the first page of the results:
- Coxiella burnetii (bonus: Wikipedia says this is "the most infectious organism known to man")
- Lactococcus lactis
- Escherichia coli
- Staphylococcus aureus
- Lactobacillus zeae
- Sphingobacterium lactis
- spores of Bacillus anthracis (that's just what it sounds like: anthrax)
- Brucella (a Wikipedia citation: Brucellosis... is a highly contagious zoonosis caused by ingestion of unsterilized milk)
- Mycobacterium avium subsp. paratuberculosis
Out of these, the Lactococcus and Lactobacillus won't make you sick. All the others are dangerous pathogens. And I don't want to go through the article you linked sentence for sentence, but in case you believe them that pathogen infections only appear in industrialized dairies (oh, and I can tell you that my grandfather is a vet specializing in livestock - the cows in big dairy farms are not constantly sick as your link claims), some of these studies were made with normal village cows in India, Zimbabwe and other places where cows are raised the same way as centuries ago.
As for the "different bacteria" point: Theoretically, the same bacteria can live in both raw and pasteurized milk. In practice, in today's industry there is practically no way that milk can get contaminated with the nastier stuff between pasteurization and you opening the carton. After opening, we hope that nothing in your kitchen can contaminate the milk with brucellas or salmonellas. Before pasteurization, any contamination can occur. So yes, the microflora of pasteurized and non-pasteurized milk is different, but it is by no means a case of "natural is better".
Conclusion: Raw milk is a good thing by itself, but it can harbor some terrible illnesses. If you want to use it, you have to be much more careful than with pasteurized milk. Letting it ferment by itself is reckless. Refrigerate it and throw it out 96 hours after milking - even a whole cistern of milk isn't worth a salmonella or listeria infection.