Hi,
I think I would have thought it would have been a learning curve to have to work in a different area of the same department. The department is not changing. Things like that can happen due to various business and department requirements. This could also be one of the clauses in the offer letter.
Also, on a positive note, see that he/she is giving another role to play within the same department, but not asking you to go to another department. We are also, at times, talking of Job Rotation; this is something similar, right? Everyone might have to get into another job to understand the pros and cons of the position, and more importantly, if you were to be a mentor in the future for the next generation.
It is better to have an open discussion, look into the context in which this is offered to you, rather than dwell on the same issue, losing your peace of mind.
Thanks