Change Of Job Descriptions, Florida

[Madisar]

I have been employed with the same company in Miami, FL for almost 4 years. I did position "A" for about a year and then a position "B" for almost a year. I then suggested to combine these two positions into one so that I could receive a higher salary and the company would save money as well. I have been in this combined position "C" for over a year. My boss just informed me that our corporate office thinks that I'm making too much money and they need to "justify" my position by adding on more responsibilties. In addition to more daily responsibilities they will be changing my job description drastically and I will also have to handle the tasks of a position that used to be handled by a full-time employee that is no longer with the company. All this for the same salary. My boss tells me that since corporate has the last word I have no chioce but to do this, otherwise, corporate could just fire me and they could hire someone for less money since they are tightening the budget. Is this legal? If I quit, would I be eligible for severance pay or could I sue them? Please help! Thanks in advance.