Is your department looking to supplement the career union personnel with part timers? Or is your department changing existing career positions to part time through attrition or layoffs? I would suggest talking to your steward or union leadership, as they should be addressing the issue if it affects your career positions. There are laws in certain states pertaining to "substitutions" or replacing full time sworn personnel with part time, contract or volunteer. The State of Illinois has a "substitutes" bill. This may have also been outlined in your collective bargaining agreement. (if you have one)
Otherwise, I will put my feelers out for you and try and get any information I can.
I'm not familiar with your situation, but it sounds difficult. I think it is important to define what you and your members provide to the public. This not only means what you do on the street, but levels of knowledge, training, and experience. This is easy to do if you have well defined training rquirements and well maintained training records. This way you can point out the differences between your members and potential part time employees. Check your collective bargaining agreement and with a labor lawyer as this may not be legal. Good luck, and keep us posted.