Sell Another Book Before the First Comes Out?

If you wait until the first book comes out, you'll have a sales record to deal with -- that can work to your advantage or your disadvantage, depending. That is the only meaningful difference, I think, and what it means varies from person to person.

I say, if you're ready and you feel good about your project, give it a go. Maybe discuss with your agent, too -- if he or she doesn't know about the big switch to YA, this is a good time to talk about it. Your agent is unlikely to tell you to hang on to saleable projects (particularly in another genre, where your sales record is less of a factor), but he or she might argue that there's something else you could do that would better enhance your career at this stage. (No idea what -- this is why I am not an agent.)

Your genre shift will actually help you here, or at least hurt you less; as an editor, when I'm thinking of resigning an author, it's always helpful to have existing sales figures (thus I'd rather wait until books have not only hit the shelves, but spent at least 6 months there). But if you're switching from adult to YA, your publication history would be less relevant, so it can be easier to make the case for snapping you up early.