I asked a question that I now realize needs to be completely rewritten (new title, vastly rewritten text, add screenshots).
Should I edit or delete and replace it?
Are there general guidelines or principles for deciding?
If a question does not yet have answers, and the revised question would be asking about the same topic/problem/issue, then it should be improved to clarify or make it more specific.
If a question already has answers, then you should not revise it if the revised question would invalidate any of the answers.
For the part about deleting: There are advantages and ramifications to deleting a question and replacing it.
advantage: a new question will more likely be seen again. While a fully revised question will bring it up in active, only a "new" one will show in the newest.
Disadvantage: Deleting questions is a no-no and the machine that runs the place will begin to dislike you :-) After too many bad questions and deleted questions, even if they are deleted by you, it counts towards your bad question score, and eventually could get you question banned. (This is hearsay I read from other questions about the machine warning/banning people, although I am sure someone else could verify it)
Adding a bounty to a question is a method to give it more visibility and put it in the featured set. Before blowing rep out on a bounty making it as readable and understandable (revised) as possible would be multi advantage. It then is likely to show on 2 lists, and it is revised to improve it.