I'm a research assistant (Wissenschaftlicher Mitarbeiter) in Germany in a BMBF funded project. I just got an offer to join a project that has a longer runtime than my current project and offers more opportunities. My professor is supports the switch. My role in the team is vital to the project (full time position) and there is no replacement for me in sight. There is a colleague who could to my part of the work but that would mean that he would have to qualify for the position (learning effort, etc.) and has less time for his role.
I'm relatively new in the position of a research assistant so I don't know how the legal situation is here. Is such a switch from a legal perspective even possible when the project is not done? How to justify the switch? Could someone dispute the switch, considering my new role was not advertised publicly?