I used to study animal management and there was a lecturer who wrote his uni thesis on “seagulls” and any time he heard someone call them seagulls he’d shout down the hallway “They’re herring gulls!”

There’s an overlap where both are valid for a few months and you can trade them in at post offices and banks but after that period is up it’s just tough luck I guess

Yeah in the UK old currency is fazed out in less than a year. £1 coins changed in 2016 and it only took a few months till the old design was invalid

That’s why old currency isn’t valid after it’s been changed though? That’s why most countries have swapped to plastic notes at the same time as adding anti forgery things.