You can upgrade or downgrade to another plan at any time in your account settings. Upgrades are immediate, while downgrades take effect at the end of the current billing cycle.
If you exceed your plan limit you will have the option to either upgrade to a higher plan or free up some resources to stay on your current plan or even downgrade to the free one.
You will be able to view your plan usage from the settings page when you are logged in.
You will be downgraded to the free plan and will be able to use the app as long as you do not exceed the limits of the free plan