If you want to downgrade your JotForm account, your data will not be affected. It will stay securely stored as long as it doesn’t go over the total submission storage range for free accounts which are 500.
However, if you go over the monthly limit on your downgraded plan, your forms will get disabled.
To check your data storage level, you can take a look at the account icon on the right side of the page at the top when you are logged in. You can also review the progress bars there.