It has been announced the new Bank of England notes featuring the King will be slowly issued from the 5th of June. The Bank of England also said a limited amount of "current or old series notes" can be exchanged for the new versions shortly after they are issued in June.
I assume the £5 and £10 will be the most common at first because those are the most used denominations while the £20 and £50 will take a little longer because those notes are used less.
https://news.sky.com/story/amp/king-charles-banknotes-to-enter-circulation-in-june-13076796
https://www.bankofengland.co.uk/banknotes/king-charles-banknotes

