Hello there, @dwcogswell.
I can understand it's vital to know how this upgrade will affect your API. That's why I want to make sure you get the most accurate answer on this. I'm sharing a link to our development team where you can ask a question. Here's what you need to do:
- Head to How to integrate with QuickBooks API.
- Click on Help, then click QuickBooks Online in the upper right hand corner.
- On the next screen click Ask a question.
Here, you can submit a question for our Development team to get back to you.
Thanks for being a part of the Community, and let me know if I can help with anything else. Have a great weekend!
Update: I did some digging and while downgrading your service and can have an effect on the API, upgrading should not. If you're talking about a custom build, we're not able to say with 100% certainty what will happen, but typically upgrading should be fine
