I signed up under a regular plan, but I am a nonprofit.

I don't want to lose the form I created. I'm going to test the product and if my submissions become overwhelming I want to be able to jump into a plan, but at the nonprofit status. Please help for the near future. Thanks.

You won't lose the form you've created if you reached the submissions limit. Your account will be in overlimit status if you exceeded the limits. It means that your forms will be disabled and no one will be able to open them.

You can upgrade your plan anytime you wishes. Here's a guide: https://www.jotform.com/help/331-How-to-Upgrade-My-Account.

If you're non-profit organization you might be interested for a 50% discount, here's the link: https://www.jotform.com/nonprofit/pricing/.  

Hope that helps. Let us know if you need further help. Thank you.