Improvements to handling add-on plan change requests as an add-on provider

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku Add-ons

### Tell us about what you're trying to solve. What challenges are you facing?

As a Heroku add-on provider, I want more flexibility in how I can respond to add-on plan change requests so I can minimize the volatility in my add-on subscriptions.

For example, I want plan upgrades to take effect immediately, but plan downgrades to take effect in the following billing cycle.