A tale as old as IT - keep the old system and replace bit by bit until you have a new system.
Trigger knew it in only fools and horses, people laughed when he said he had changed all the bits of his broom but it was a new working broom.
HMRC do something similar - they lost their case against Kay Adams and kept taking it to tribunal after tribunal with all the assets in the world to pay for all the barristers they like...