I have not tried Steve's suggestion yet on correcting this 2020 problem.. mainly because I'm not sure what you mean by

"Honestly, I'm not sure whether it will purge historical transactions - maybe someone else can confirm this, or you can test it out and see."

If it does purge historical transactions, exactly which historical transactions will it purge? Only for these 2 customers? If so it's not a problem. If it's going to purge all of our customers transactions this would cause a problem.