Unable to change build attached to version in App Store Connect

I have an app version (1.2.1) in App Store Connect that is showing "Ready for Review" status. It has an old build (1.2.0, build 4) attached to it. I uploaded a new build today (1.2.1, build 5) and need to swap it in, but there is no "+" or "-" button in the Build section of the version page, and clicking the current build number just navigates to its metadata page. I have tried:

Creating a new version (1.2.1) — it auto-populated with the old build Uploading a new build with the correct version string — still no option to swap Looking for a "Remove from Review" button — it does not exist on my page

Is there a way to detach the current build and attach the new one, or do I need to delete the version and start fresh? If so, will that affect my existing review thread with Apple?v