What happens if I add a custom skill to a digital assistant, and then upgrade the digital assistant?

There won't be any impact on your custom skill. When you upgrade the digital assistant, the upgraded version will contain the predefined and custom skills that you already added to your digital assistant.

Note: When a digital assistant is upgraded to the latest platform version, the custom skill will need to be in the same platform version for it to be added to FADigitalAssistant.