Can't update available backends for PR previews

**Is your feature request related to a problem? Please describe.**
I initialized the "Previews" feature for our Amplify project with three environments "develop", "staging" and "master". After some time we decided to switch environments. But the dropdown menu to "Point all Pull Requests for this branch to an existing environment" doesn't seem to update with our new environments. Is there something I can do there?

**Describe the solution you'd like**
Either an automatic update or a manual update via a button.

**Additional context**

No files or tests are named. Start by tracing the Amplify Hosting Previews settings and the dropdown that assigns pull requests to an existing environment, then reproduce the environment-switching scenario described here. Done means the available environments update automatically or through a manual control and the dropdown reflects the current environments.

Written by the indexing model from the issue text.