I've been responsible for a particular department/product within my team for over 9 months now.
In this time I've really learnt the role and understood the business. I love what I do because it keeps me on my toes and it's a constant challenge.
I've also built some fabulous relationships in this time and some of them are very senior members of the department.
I've written procedures, I've amended processes to make them more efficient and for lack of a better phrase the work became "my thing".
I'd just reached the stage where clients and senior members of the management would call and ask specifically for me.
And then my direct boss (who hates me) dropped an ultimatum- I was being moved to another department within the same team and I am no longer "allowed" to touch any of my previous work and instead train someone new and hand it over them to them 
This is killing me. I don't want to be moved, especially because the new product I am being given doesn't interest me at all.
I am feeling frustrated and cheated. I don't want to let go of my work and hand over the stuff I've worked so hard on these past few months. More than anything else, I don't want to lose the relationships and contacts that I have made and the idea of someone else taking my place is unbearable atm.
Any advice on how to handle this?