What happens if a Starter package user exceeds the 100 submission limit?

The topic says it all really. With a couple more questions.

Does JotForm inform the user when the submission limit has been reached, perhaps with the option to upgrade?

Is it possible to upgrade for the three months the forms will be in use and then downgrade for the following 9 months when they're not and then rinse and repeat?

Thanks.

If the free subscription plan exceeds 100 submissions limit, the forms shall be disabled and it is required to upgrade the account. 

Does JotForm inform the user when the submission limit has been reached, perhaps with the option to upgrade?

The user will be notified by email and a notification will be displayed on your account to upgrade your account when the account has reached 80% of any of the limits for the account. 

Is it possible to upgrade for the three months the forms will be in use and then downgrade for the following 9 months when they're not and then rinse and repeat?

You may upgrade and downgrade your account as per your requirement. However, please note that when the account upgraded/downgraded respective plan limits are applied automatically to the account. If you have more submissions than the Free plan, when you downgrade to free you'll be required to delete the submissions to clear the space.

Hope this information helps!