I have sold one picture book that is around 700 words. The rest are much, much shorter. Some are under 200.
I have written early readers, as well. They were submitted in the same way that pb texts are submitted. But early readers are a totally different thing. It would not be easy to convert a pb into an early reader. Generally speaking, I wouldn't recommend it. There's much more to it than word count. Also, early readers are much harder to sell, particularly as a new author.
I'm not an expert on MG but I suspect that converting a PB into an MG would basically be impossible. I mean, maybe you could use a seed of the idea.
I suggest you read lots of picture books and take notes on the ones you really like. You can look up their word counts. It's also a great exercise to type out the text in order to get an idea for how it flows with the page turns, and how it looks without illustrations.