The specific example is:
The answer by hobodave is quite thorough so I was wondering, would it be alright to rewrite the question to be more general, say "Can any harm come of eating fruit seeds?"
I don't think it's a bad idea. I was thinking that might be useful as I was answering. I could see someone asking specifically about apple seeds in the future, given the rather widespread pseudo-legend. I'd rather answer once thoroughly than a simple "yes" or "no" each time.
Generally as long as you don't change the question so that it renders answers completely irrelevant it's OK.
I don't aim for any particular level of omniscience in my answers, so I have no personal qualms with any edits you make. Feel free to tweak my answer as well if needed post-edit.
The general rule of thumb is it's OK to do what you think is beneficial until told or asked otherwise. So, if you edit the question and the OP doesn't like it, and reverts it, just leave it alone and don't edit his stuff again. It never hurts to leave a comment explaining what you're did and why. (I prefer this to the asking of permission in every case, less noisy comments.)
As long as you are certain (or at least strongly believe) that your edit:
...then go for it. With respect to #4, you don't need to ask permission every single time, but if you see that your edit has been rolled back, don't keep re-editing.
Point #3 is probably the most important one. 15 minutes ago, there was only hobodave's answer, so I would have said, let 'er rip. But now a new answer has been added that makes specific reference to watermelons, so changing watermelon to just "fruit" would invalidate that answer. So as of right now, I would not make this edit.
Anyway, don't worry that the question will be difficult to find because of its specificity. It's tagged
[fruit], so it'll still show up in the Google results if somebody searches for that instead of "watermelon".