I have a SD Card wich works perfectly on my Pi 3 B+ but I want to use it in my other Pi 3 but the Pi doesnt seem to start.
Is there something that needs to be changed to use the same Card for both versions?
No rep to comment yet, so this goes as answer.
This is generally a bad idea to use the same SD card swapping between devices, as each of them has its own configuration - despite the same system. Devices differ by on-board components id, or even might be different due to newer revision of board. I own 2 rPi 1 B+ and swapping cards is impossible due to errors with boot.