There are changes but it's not the kind of changes people think about.
Mewing doesn't change your bones and doesn't change your muscles, what it does change it your natural facial posture.
If you haven't 'mewed' before, your natural posture will be that mouth breather look everyone talks about. Your jawline will be weaker, your hyoid will be lowered, etc. But mew for long enough and your natural posture will change as if you're always mewing even when you aren't. This happens without any changes in your actual facial features.