There's Still A Handful Of King George VI Postboxes Out There.

https://us.cnn.com/2022/09/26/business/king-charles-iii-queen-elizabeth-ii-coins-banknotes/index.html

So I think it's a fair bet that Queen Elizabeth II currency could circulate alongside King William V currency.

Unless Britain just completely gives up and adopts the traditional mix of US Dollars and toilet paper.