University of Missouri

With MU in discussions to buy St. Mary's Hospital, will benefited employees (Part-time and full-time) who are retired from the state of Missouri and receiving their retirement benefit have to stop that benefit when they become MU employees and fall under that plan? 

No. MOSERS and the University of Missouri are separate retirement systems. A MOSERS retiree could take a part-time or full-time position with an MU employer and continue receiving their benefits since such employment would not be in a MOSERS or MPERS benefit-eligible position.