I suppose that whoever issues the banknotes (Bank of England/Scotland) already has a schedule for replacing old worn currency and printing new ones. The new ones will now presumably have King Charles' portrait on them, but the older QE II ones will remain in circulation and remain legal tender. It's not like everyone's money suddenly becomes worthless or that they have to trade it all in for new money. Coins have a way of surviving for a long time, much longer than paper currency, so the QE II coins will be in circulation for decades yet. People will just find fewer and fewer of them in their pocket as time goes on.