New short-term identification – Question about the journey without HU

My question: What happens if you use the license plate instead as before? Is the license plate abuse or just driving without HU? The valid TÜV for the new license plates is not recognizable from the outside anyway. I assumed that the SVA would stick a badge on it. Also the TÜV does not stamp the new license plate – which looks like a normal one – because the company is not able to use the license plate. hrzeug is not allowed. Plaques are also not glued. Would just interest me, because I actually have a youngtimer that I have woken up from sleep from time to time.