Welcome to the Community. I'm here to assist you with your invoice concern in QuickBooks Online.
If you're referring to the old display when creating an invoice, the option to change it to the old one is unavailable. Users with a new subscription will automatically have the new layout.
At this time, you can only change the template selected to personalize the content of your sales form. If you meant changing the template back to the old one, please refer to this screenshot for your visual guide:
In case you're referring to something else, let me know. You can include a screenshot or additional details below.