Hi
I think, i would have thought it would have been a learning curve to have got to work in different area of the same department. The department is not changing. Things like that can happen due to various business, department requirements. This could also be one of clauses in the offer letter.
Also on a positive side, see that he /she is giving an other role to play with in the same department, but not asking you to go to other department. We are also at times talking of Job Rotation, this is something similar right, where everybody might have to get into other job to know the pros and con's of position and moreso, if you were be a mentor in future for generation next.
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