It depends on the formula you choose. Typically when you have a jointly governed plan there's attention to avoiding big swings in the contribution rates. Nobody likes that.
If I understood your question correctly, you were concerned about the people whose pensions are already being paid or people who are about to receive it. Typically when you do a transition like this in a controlled way, you're grandparenting the people who have already gone through the system.
You would certainly encounter a lot of opposition, I think quite reasonably, if there were anything in prospect that would cause the value of benefits and pay, or benefits that have largely been earned or entirely earned, to go down. The trick is to make the transition, as happened in 2000, in a way that leaves people intact. You phase it in. It's not quite fair when you think about the status of the newly arriving public servant versus the public servant who's about to retire.