This is quite subjective in my opinion.
Where is the balance between: - modifying existing, outdated and obviously unpopular question to better suit your needs ( but also stealing some existing theme) and - starting your on similar question and risking a duplication marks?
Well if the problem reoccurs for the same situation, you might be able to edit your previous question. Let's take Stack Overflow for example and assume that I have a question on Python's
int() function with no votes casted, earned the Tumbleweed badge, and no answers. If my new problem occurs with the same code I presented in my post, I can add that question to my post.
Now let's say I have different code, I wouldn't be able to do that since different code means different characteristics and different factors to take in consideration. If I were to edit my question, I would create a similar but drastically different post based on its contents. So maybe the question in mind is similar, but the situation at hand and its factors and characteristics allow it to be an entire separate post.
Like I said, this is subjective so there may be cases where I can be wrong (i.e editing the question will provoke others to rollback and suggest you to make a new post or making a new post that is marked duplicate of your previous post).
If the situation is different (i.e code as in the Stack Overflow example), make a new post
If the situation is the same, edit the post to include your new question
If they have something in general but are different, also consider editing your question for clarity or adding a bounty to attract more attention
In the end, the decision is yours to make anyways.
the fact that old, unanswered question could be someone else's, so my dilemma refers to "stealing" the question.
Google is your friend. If you searched all over Google related to your problem and search on the site your post is in and still can't find an appropriate answer to your problem, you should have no worries whatsoever. If a duplicate is found, simply delete your own question (if it has no answers) and make sure to view the duplicate.