Can I edit/switch old entries into a new or different "class", or will that mess up reporting or financials?

Welcome to the Community, ccpres.

You can edit records in your books and change classes as necessary.

Here's how to edit classes in the Product/Service line items on an invoice:
 

1. In your left navigation bar, go to Sales, then Customers.
2. Find the customer you're working with and click their display name.
3. Locate an invoice you want to change a class on and click its Edit option.
4. From your Product/Service line item area, find the CLASS column and make your necessary changes.
5. When you're finished, select Save and close.

Here's how to edit classes on transactions in your Chart of accounts:
 

1. From the Chart of accounts screen, locate an account you're working with and click its View register option.
2. Find a transaction you want to change a class on and open its record.
3. Using your CLASS drop-down list, make the necessary changes.
4. Select Save.

In regard to if changing a class on records can mess things up in your books, there will be differences after making any change. To properly determine if changing a class on a particular record will mess up reporting or financials, I'd recommend working with an accounting professional. If you're in need of one, there's an awesome tool on our website called Find a ProAdvisor. All ProAdvisors listed there are QuickBooks-certified and able to provide helpful insights for driving your business's success.

I've also included a detailed resource about working with classes which may come in handy moving forward: Create & manage classes

I'll be here to help if there's any additional questions. Have a wonderful Wednesday!