Once your testing install eventually becomes stable, in this example when bookworm was released it became stable (so you again had to change sources to trixie) and the next new testing branch is named, you must be careful before upgrading to the next testing version.
First you must ensure you have no orphaned packages: Checking for packages that are removed from testing that you would be unaware of Follow that and ensure you are clean before continuing.
So check the “Maintaining a Testing install - Checking repositories/packages installed are clean” page (above link) and ensure you are in a clean state before upgrading, so you do not have issues.
Next ensure you have rebooted and are ready to upgrade, and that a timeshift snapshot is in place. This way you can revert if there are any issues and not worry about things going wrong.
Once you have that all ligned up, you can upgrade to the next testing version once again :)