A writer friend of mine commented to me once that after the sale of your first work, publishers will expect you to stay in that genre – if your first is a YA those that follow should be YA. I’m currently sending my YA novel to publishers and an agent or two. (Will be my first once sold.) Everything else I’ve written is MG. Is my friend right? Will I have a hard time selling my MGs if I sell a YA first?Some of the best idea people work in more than one genre and age group, and publishers know this. M T Anderson, Adam Rex, Rosemary Wells, Ann Martin, Jaqueline Woodson, Dav Pilkey, Kevin Henkes, Brian Selznick, sheesh, the list goes on.
You may have sold your YA novel to a publisher imprint (or editor) who really only does YA. In which case you'll have to send your middle grades somewhere else. But real artists cannot paint themselves into a corner. For real artists, there are no corners.