How can I copy a custom form style invoice so that I can create another with only one small change?

It's nice to see you in the Community again, @communityoil.

I'm here to help share some information about customizing form styles in QuickBooks Online.

Currently, creating a copy for your customized form is not available. You may need to create a new style if there are minor changes from the previous template created. 

However, you can use the import feature to copy an invoice template. You can make your own invoice design and copy it from one document to another. For the guidelines and steps, please check out this helpful article: Import custom form styles for invoices or estimates.

That should do it. For another related article you may want to use in the future, you can check out this helpful link: Customise sales forms by location.

Let me know if you have other questions about your forms. I'm always here to help.