Yep.
Coinage will take years to be replaced.
Notes will already be designed in the current template.
But might need amending. They will be dribbled in over the next few years.
I reckon we will see the first King Charles money on £10 notes and £1 and £2 coins.
Stamps will see a more rapid deployment. 1st and 2nd class available for the Christmas post.
Because of counterfeiting measures, notes are very complex and expensive to print. So natural wastage will come into play. The £10 note is the most used, so that will see the first replacement notes.
Stamps are easy and cheap to print, and the decision to scrap a load of already printed items and replace them with new Charles III ones is easy.
They've been running out plastic notes (£20 & £50) ready for the phasing out of the paper ones by the end of the month, so they will possibly have an enormous stock with Elizabeth on them. As the plastic ones have a much longer life (including £10s and £5s of course), their "natural" disappearance from circulation will no doubt take years longer than the paper ones did. So presumably we will have mixed notes for much longer than we did in 1952. Unless the plastic ones are withdrawn well before their natural decay - I don't know whether they can be recycled, being plastic.