I am a kid-lit author with several published MG and picture books to my name. I have also published a couple of YA novels for younger YA readers. I love writing for young people, but the new book I am working on is very different and will definitely be for adults.
I'm wondering if you know anything about how to transition to this new audience without having to use a pseudonym. With such an extensive publishing history, I don't want to lose the name recognition even though I'm targeting a different audience.
Darn it! You've already eliminated the most obvious answer here - to use a pseudonym. But I understand your reasons for not doing this and respect them.
Firstly, I would ask if you currently have an agent representing your kidlit projects? If so, do they also represent adult books? If they do, they will be in a far better position to guide you through this transition than I. But given you've asked here, I suspect you either don't have an agent or your agent does not represent adult books.
So... If it's the latter, you need to talk to your current agent. Let them know that your new book doesn't fit in the wheelhouse of your previous work. Depending on your working relationship, she may be willing to explore new avenues on your behalf. Or there may be another agent at the agency who could represent your new project and the two agents could work together on your career.
If you don't already have an agent, or your current agent doesn't have any leads for you, you will probably want to query your new book when it is ready. Research widely for agents best suited to your new work. Make sure that when you query you let the agents know that you have a body of work and (if it is the case) and agent already representing this facet of your career. This will give you a leg up on authors that don't already have a track record of publishing and working with an agent.
Obviously the whole querying thing is a lot of work and if you feel like this book is a one-off and you probably won't write another adult book, you may not want to do this. It may also be harder to get an agent for the project because agents tend to like working with writers' careers, rather than single books. If this sounds like you, there is always self-publishing or even working with a small press that doesn't require an agent to submit.
There are so many variables!
Hopefully this is helpful.
Best of luck with finishing the new book.