What on earth does this have to do with Worker's comp? Unless they did this demotion to you after or during a worker's comp claim, which would really change the complexion of the whole issue. So if that's the case, please tell us.
Otherwise, the answer is yes, they can demote you for any reason. What your options are is to quit and file for unemployment benefits,but it pays a very low max weekly benefit in Mississippi anyhow, and try to find another job. Your chances of being approved for this low weekly benefit amount are fair to middling, provided you have not worked at the demotion job yet when you quit. If you work it (even for a day, an hour, a minute)and then quit, you will have very little chance of being approved for unemployment.
Or you can keep working the new job until you find another job. But you can't sue them or protest to the bureau of fairness because there isn't one, and MS has very few labor laws and is an at will state. So they can do whatever, as long as they did it not because of your race sex religion, age, all those EEOC issues, you don't have much you can do about it.