Opinions on Re-Querying Agents

November last year, I started to submit my fantasy YA novel to agents. I freely admit that I committed one of the major cardinal sins of writers - querying too early. Since then, I have editing and polished the MS until it shone, with the help of some very talented beta readers. The story is much stronger now than it had been back in November. I honestly can't think of anything that I would change in the story.

That said, I accumulated a lot of rejection letters from my first query-go-around for this novel, many of which were just on the query letter itself, without a writing sample. I also revamped the query letter. It's much stronger now.

So my question is, is it all right to re-query agents? I was only planning on re-querying the ones who rejected the story passed on the old query letter, although I could mention that the novel has been revised since they had last read it. Should I mention that they already rejected the novel?

Querying agents is hard work, and despite all of the agents' blogs that I read, and authors who have agents, I can't help but have some questions. Not that I think there is one right answer for everyone.

And for those that are curious, here are my query stats for my other novel, Hidden in Shadows:

queries sent: 40
rejections: 11
partial requests: 4 (2 rejected)