Hey there, ava724. Welcome to the Community. You've come to the right place to get this figured out. I'll help you with this.
If a plan is downgraded the current transaction records will still remain. However, recurring transactions for example aren't supported in Simple Start. If you wish to switch to that specific plan you'll want to turn your recurring transaction templates off before you make the change, and be sure that you have an alternate way to track your scheduled transactions.
I have an insightful article I'd like to share with you which covers what happens to transactions (both normal, and recurring ones) when you do this. It also details what differences there are with other things like users, inventory items, multi-currency, etc.
Here it is: What happens when you Downgrade your QuickBooks Online plan
If you have any further questions, you can always reach out to me here. I'd be happy to assist!
