I think it depends on what you’re trying to change. My Loved One does not cover things before she puts them into the refrigerator. I could change that, no doubt about it. (But, though it drives me crazy, it’s easier just to wrap the things myself.) It’s easy to change people in minor ways, and the more they have vested in you, the more they care about you and what you think, the easier it will be. (It is, of course, often worth reflecting on whether or not you actually want the changes that you think you do.) There are lots of ways to make minor changes in people - some that are almost unnoticeable (ex: reward-based conditioning, when done correctly) and some that are very obvious, not to mention abrasive (ex: making such an enormous and terrifying scene that people will change at least their behavior - not necessarily in the way you want, though - to avoid a reoccurrence).
I get the sense, though, that you’re talking about fundamental changes - making an irresponsible person more responsible, for example, or making a very methodical and pragmatic person free-wheeling and zany.
Can it be done? Well, yes. We’ve all seen cases like this - becoming a parent, for example, can make a person much more responsible. Sometimes.
Can it be done deliberately, with forethought? Yes, though that is much rarer. It requires a lot of intelligence on the part of the person instigating the change, and a lot of cooperation on the part of the person doing the changing, not to mention a lot of effort from both people. Therapists sometimes manage it. Spouses sometimes manage it. Parents sometimes manage it. Most often, though, it isn’t exactly changing someone, it’s facilitating change, which is a different matter; the difference is the crucial one of where the change originates. Changing someone against his will is very hard, and very rarely successful, though that also has been done.
Can you count on it? Absolutely not. Not even the most successful professional can count on being able to change another person in a fundamental way; every therapist, for example, has unexpected failures as well as unexpected successes. A person making a first attempt at changing someone - say, a person who went into marriage thinking, well, I’ll just change X and Y in my spouse and then we’ll be fine - is very likely to fail. Better to count on not being able to change anything.
Should you try it? This is a much tougher call. I would not try it. It is dangerous, it can be unethical, it’s rarely in another person’s best interests, and it feels like hubris. When you change something fundamental about a person, you change other parts of him, too - that’s inevitable. You may end up with a situation worse than before you started. And, more importantly, I don’t think I, at any rate, have the right to try to change who someone else is. I’ll live with it if I can, I’ll get out if I can’t - but I don’t expect the world, or any person in it, to build to suit. Also, I think you need a lot of confidence, more than I’ve got, to wield that kind of power over someone else.
You also have to think about the consequences of your actins, and why you’re undertaking them. For every action there is an equal and opposite reaction. I’m not saying this is absolute law when it comes to human personalities, but I do think that you risk changing yourself as much as, or more than, you change someone else. This is especially true if your motives are less than pure, and in a personal relationship, they almost always will be. Do you want your spouse to be more aggressive and energetic in his career because he’d be happier or because you’d both have more money? Do you want your spouse to exercise and diet because he’d be healthier or because you’d like the way he looks more? These are just examples, but you see what I’m trying to say - there’s often an ulterior motive. If there is, I think you are risking even more harm to yourself, as well as to the other person, by trying to change him.
If someone in my life truly needed to change, say because his current path would almost certainly lead to an untimely death, I wouldn’t try to facilitate that change myself. I’d encourage that person to seek help, not so much because I have some kind of faith in the divine abilities of therapists, but because a) the act of seeking help is a very important first step on the road to change and b) I would not want to have that relationship - that of changer and changee - with someone I love. It’s not what I expect of love or what I want from my life.
Wow. That’s long-winded, and I’m not sure if it makes sense; I’m pretty tired. Basically, I’m just saying - yes, you can change some of the people some of the time, but it isn’t reliable. And most of the time it isn’t a good idea.