This fairly simple situation has been turned into a mess.
Yay, fake facts for the weekend! It was never a simple situation and only those hell bent on getting Britain to hell in a handcart claimed it was. Those are the real saboteurs and anyone who claims that True Brexit™ is somehow being sabotaged are the true enemies of the people.
Whatever deal was negotiated with the EU was, it was always going to mean massive changes to lots of IT systems. But until you know what this will be, you can really start the work. Chaos at the ports, airports, at the farms, etc.