I would not advertise this account as free if there is a fee to get monthly paper statements; since what you’ve described sounds like you would be adding a monthly fee for statements that didn’t exist before it would likely be an issue. Using “free” always comes with risk, more now than ever, and I’m not sure it is worth the risk – Reg DD and UDAP (IMO). If you could grandfather the existing accountholder where there were no fees (including no paper statement fee) and create criteria going forward (that has clearly disclose criteria, timeframes for completion, and action/costs that result if criteria aren’t met (e.g. converting to new account with new account disclosures given XX (30, 60, 90 days) prior to change) it reduces the risk.
While you’re making these decisions take a look at this enforcement action from the CFPB: https://www.consumerfinance.gov/about-us/newsroom/cfpb-takes-action-against-mt-bank-for-deceptively-advertising-free-checking/. Keep in mind this has been on the regualtors’ radar for years and is a very easy to identify issues.