Thank you for your response, Paul. I updated my earlier message to correct the comparison from defined CONTRIBUTION to defined BENEFIT, as that was the terminology I intended. I haven't calculated or been shown what my benefits would be under a defined benefit plan, so perhaps the cash balance plan might still be the better option!
I'm asking about documentation retention requirements because I had always assumed I was part of what employees referred to as "the old plan" -- a traditional defined benefit plan. Now that I see that's not the case, I'm curious about what documented proof exists that would confirm plan changes were approved by me.
Given the extensive documentation and signatures required for any serious financial matter (changes in spousal beneficiaries, opening accounts, etc.), I would expect there to be some form of proof of informed consent and authorization. This might include, but not limited to, documentation showing physical or electronic (notarized?) signatures.