No, you upgrade apps individually—create as many free apps as you'd like, and upgrade just the apps that need more features or support.
Your app will downgrade to free at the end of the billing cycle (e.g. at the end of the current month if you pay monthly) and you will not be charged again for that app.
You can upgrade again if you change your mind.
No, but anyone can create an unlimited number of free apps with Glide.