The current iteration of the AGR credit card was only available since ~2016, since the Chase to BoA change didn't have the account move over (a new card application was needed.) There wouldn't be 20 years of never a missing a payment as far as the AGR card is concerned.
Also, unless the AGR card was a large portion of your available credit, and you're using other cards heavily, or it's one of your oldest accounts (which, if you've had 20 years of never missing a payment, should mean you have a 20-year credit history with a 20-year-old account) it shouldn't be a large ding to your credit. The closed account stays on your credit report
for up to 10 years, so for average age of accounts it won't even affect you for a number of years.
In general, the transition didn't seem complicated when reviewing the literature sent out. My AGR credit card account was moving from BoA to FNBO on 10/23, and other than some housekeeping items like setting up autopay and making sure to grab old history my account was essentially the same, just with a new credit card number and new bank. If I closed the account before the transfer, the account wouldn't transfer over to FNBO since the account was now closed out. For most people, this is the best scenario - no worrying about whether you'll be approved for the card at the new bank, no having to get another hard inquiry and a new line of credit, etc.