This question is a huge can of worms, and would require a book to answer adequately. Yes, there are things in the conformation of a young foal that do not normally change, hip angle would be one of them. Neckset is another. But many other things do or can change, sometimes radically. Sometimes a "reason" for the change can be identified (eg a nutritional problem or inadequacy, or an injury effects development), sometimes no "reason" can be identified, (it just happens... bad luck). This is the "crapshoot" of buying young horses, as it is also in breeding horses. Nothing is concrete, no guarantees. When the process works well, one congratulates one'self as if one has a clue about this sort of thing. When it doesn't work out as well as hoped, that is the downside, and reality, and what we might consider a "conformational flaw" or "imperfection" is the result, even from the best and most accurate and well researched breeding program.