Over the last few months, I've been working on integrating ArcGIS maps into my app using the JavaScript API. I'm about to publish the app, and decided to make it legal. My understanding is that I needed to have an ArcGISOnline account and some credits to be legal. I will also begin using the REST API this week, which costs credits.
what, if anything do I need to change in my code now that I have an account?...how would anyone even know I'm accessing the API legally?
As far as I know, you don't need to make any changes to your code. I think the legal threat would only be raised if you were doing something evil with the API.
Basically, you've just purchased a license, which you can show if anyone asks.