Was it necessary before changing lanes at the intersection to make sure that it was safe?

Yes, changing lanes is a manoeuvre that always requires special caution. According to Art. 22 para. 1 of the Road Traffic Act, a driver may only change lanes after ensuring it will not cause a hazard. In practice, this means you are obliged to check your mirrors and the so-called 'blind spot' to avoid forcing another driver to brake sharply or change their course.