can they turn my job into part time and hire some one else for the other half of my job

After working for 9 years full time, my ex-company told me that they were going to turn my position into two part time
jobs and offered me one of the 4 hour slot which I took, then about 1-1/2 yrs later they laid me off and gave me my last pay check and said they will pay me severance pay for 6 weeks, but they are paying me severance at my part time wages and have not prorated their
severance payments to include some of the full time years. I've read your at will and severance topics, but I am not sure that it was legal for them to give 1/2 of my job to someone else.. plus they laid off other part time people, except for one part time employee whom they offered a full time position, shouldn't they have asked people with more seniority, that they were laying off,if they would like that position,or does the at will explain their right to do anything? Any info you could offer would be greatly appreciated.