Can a company force an electrical engineer to work in Mechanical dept

I am working as electrical supervisor in quality control department of a government owned company but registered as private Ltd located in Nagpur. I have completed my education in electrical engineering and have 15 years of experience in electrical stream. Now management want me to work as mechanical supervisor too due to manpower shortage and learn mechanical systems that to without increasing any pay or allowances. When I refused to do so, my boss threatening me to transfer me to another department or another plant of company. I am settled in Nagpur for last four years. Now I don't know how to react. Is it legal way for management to force like this. Please guide me, shall I bend to their pressure or not to accept? What actions can I take if mgmt forcefully changes my department or transfer me to other locations? My offer letter mentions my post as electrical supervisor (quality control) in Nagpur office. But it also says it reserves the right to transfer or assign other job.