Job to change on return from maternity leave: ask the expert

I will be returning to work in March after a year of maternity leave. I have been with my company for five years and this is my second maternity leave. During my maternity leave my responsibilities (managing a fund) were taken over by a colleague. The colleague who provided maternity cover had the capacity to do so as the fund he managed was in decline.  I had an informal meeting with my manager in November and she mentioned she did not want to separate the management of the two funds again, and didn’t know what I would be doing on my return. Last week, another colleague told me she had heard I will be returning to a different role. I am aware that there is some ‘new business’ that has been won in my absence. My question is would it be reasonable for my manager to reallocate me to the new business on my return? My preference would be to continue with my previous role, as I think would be significantly more interesting. And if I am told I am doing a new role upon my return, how should I deal with this situation? I would obviously state what my preference is, but given others on my team are already discussing changes to be made have a feeling my preference will not make much difference! I can completely see from a practical viewpoint why my manager would rather the management of the two funds not be split up, but had I not taken maternity leave I am fairly sure I would have carried on managing the same fund, with my colleague who had capacity taking on the new business. It is a small team I work in and there is no HR person. My manager reports to the CFO based in New York, and I do not have any sort of relationship with him, so am unsure as to what procedure I should follow.