If you have already filed your 2019 tax return the bank routing and account numbers cannot be changed.  You can change those numbers on your 2020 tax return that you will file in 2021.

So then will a paper (2nd stimulus) check be sent to those people who’s account numbers have changed since 2019 tax returns, or what happens then?

No, it looks like the IRS will not be issuing paper checks to those people whose account numbers have changed.

According to the IRS, "If the second Economic Impact Payment was sent to an account that is closed or is no longer active the financial institution must , by law, return the payment to the IRS, they cannot hold and issue the payment to an individual when the account is no longer active. The IRS advises people that if they don’t receive the full Economic Impact Payment they should file their 2020 tax return electronically and claim the Recovery Rebate Credit on their tax return to get their payment and any refund as quickly as possible."