Thankfully we’ve been down this road before.
Somewhere in the depths of the Mint or BOE (I can’t remember who designs them) there will already be a selection of designs with Charlie’s face on them, Will’s too and probably a couple of others should the unthinkable happen.
Nothing when it comes to the Palace is rushed or course, but within 6 months of Liz tipping face first out of her throne these new designs will be announced and put into production – they will probably avoid the temptation to redesign them, they’ll be the same as the ones we have now with the next Chief Lizard on the front for continuity in case the shock is all too much for us muggles.