Query question: requerying after major revisions

 I recently completed writing a thriller, and began querying it as such. After a few rejections, I began to reread and rethink about the book, and it seems as though (with a few very small changes), it would have been better as a young adult thriller. I changed the age of the main characters to better reflect that, and edited a few bits of the story, and it is now about 100 times better.

Is it acceptable to re-query agents that had already rejected the original version of the novel? What about querying a different agent in the same agency (one that, obviously, deals mostly in YA topics), even if I haven't heard back from the original agent yet?

What you're asking is if it's ok to re-query after a MAJOR revision (this revision changes the category of the book--that's major.)  You certainly can, but this is info you need to lead with in the query.

Here's what that would look like:

Dear Snookums,

I've revamped my novel SharquesGoneWild from an adult to a YA thriller.  It's a lot better now. I hope you'll want to take a second look.

Obvioiusly of course, not those exact words but you get the idea.

And yes, if you haven't heard from an agent, it's ok to query someone else at that agency.

This is the kind of thing writers obsess over endlessly and often think "oh my god, I'll end up on the blacklist of all time if I do anything remotely off the guidelines" but remember: agents are looking for good work to sell. All the huffing and puffing I do about format and query madness is to help you show your work in the best light. [It's easier to see the work is good if the lights are ON.] If this book is good, we want to hear about it.