Currently, the best way to change testing branches on a single package is to switch the entire system to the testing branch in question, do a full system update, and then go back to stable (or whatever) and skip the change on the package.
This is not a great way to do it. I’d be better off manually downloading and installing the package.
I understand that the reason this isn’t an option is to avoid package conflicts and dependency errors. This makes sense for kernel packages and libraries, but does not make sense for packages that have no dependencies.
For example, I want the latest version of go (1.15.7-1), but I don’t want to develop my go applications in an unstable environment. Go has no dependencies and is thoroughly tested by Google before release. In this use case, being able to switch just this package to ‘testing’ or ‘unstable’ makes sense.