Yes, a police officer can be upgraded to become a senior
officer. However, don't expect to be automatically upgraded to SO
after you got your degree! You have to apply, get endorsed by your
supervisor, and attend interview by a interview panel made up of
very high ranking officers. Only a small portion of graduate POs
who applied get upgraded each year. You also cannot quit the force
and apply as a senior officer. You will not be accepted. If your
ultimate goal is to become a senior officer, get your degree
1st.