Since I don't know what state you are in, I can't give you a definitive answer to your question, but here is some food for thought:
In many states, public pensions are viewed as contracts with the employees, and so stated in legislation, and in some cases in the state constitution. Any reduction in benefits for current employees is typically prevented. Future tiers may be added for new employees with lower benefits, with no effect for current, unless employee's opt for them. The DROP program may be method of jawboning employees into a new program.
Check past labor contracts for MOB (maintenance of benefits) provisions to provide precedent.
See if your local EEOC folks might have something to say about reductions for older employees.
Get your labor counsel to check with your state legislators to see if this type of move is legal under any of the above circumstances, or whether there are other circumstances that would prevent such happenings. If you have Labor Counsel, I would be happy to discuss this with them.
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