Connie ~
You have my sympathy. This is a terrible situation to be thrust into by your employer. They are basically maneuvering you into a position where you're unable to do the job they now require. Under that scenario, the employer can legally dismiss you for not being (physically) able to perform the job. Of course, this wasn't the job you had and could perform just fine, but I believe that the law allows the employer to determine the type of job you'll be required to perform, as long as it's within the confines of the kind of work you were hired to do, i.e. you can't require a receptionist to load trucks in the warehouse or the warehouseman to work in sales unless you request those jobs and even then, the employer has the discretion as to whether he'll honor that request.
In your case, this is a wholly discretionary decision by your employer. It may be simply a necessary re-assignment of duties that often occurs in business - an attempt to utilize you more effectively or replace someone who left - or a ploy to ease you out of the company by putting you in a position you cannot fill due to your SSD. You need to talk to someone in the Human Resources department but if you have (as I suspect) and they were no help, you'll want to consider going here
http://www.ada.gov/ to find out what your options are. I would be sure to let the HR department know that you're researching your options via the ADA. This may motivate the company to reconsider this move - but don't count on it. Hiring an attorney should be a last resort, but not beyond consideration, should all else fail. With the ADA law in place, this shouldn't happen. Unfortunately, all too often, it does.
If you can't perform the job the company has offered, it'll be very difficult to qualify for unemployment benefits because the company can claim they fired you 'for cause'. In their version, they'll claim that you couldn't/wouldn't perform the job required of you. Should you have to look elsewhere for work, being dismissed from your last job 'for cause' will also hurt your employment prospects when you seek a new job. This is terribly unfair, but as President John F. Kennedy once remarked,
"life is unfair". Nothing has changed in the years since the young president made that comment. There is always the option of trying to fill the 'new' position, but that is likely not feasible. Just a thought.
I don't know if anything I've posted will be of any help to you but I hope it will. If not, I trust that others can supply the kind of advice you're seeking and I sincerely hope this situation can be resolved in your favor.
Jim