Royal Mail wants people to use up stamps that don't have a barcode by January 31st because they will no longer be valid for postage.
"everyday" stamps featuring the late Queen's profile are affected by the deadline.
The introduction of barcodes made deliveries more efficient and improved security.
After the deadline, customers will be able to use themed stamps.
Anyone who can't use older everyday stamps by 31 January will be able to swap them for newer barcodes for free.
The change of monarch doesn't affect Royal Mail's advice. Further information on the launch of stamps featuring King Charles will be made after consultation with the Royal Household.
Royal Mail introduced barcoded stamps in February.
People will be able to watch videos, find out information and send birthday messages to each other through the barcodes which can be scanned with the Royal Mail app.
The analysts at Mintel said that it was indicative of how widespread smartphone ownership had become as well as people's familiarity withQR codes, particularly during the Pandemic.
The postal service reminded people to use their non-barcoded stamps by the deadline.
Royal Mail has placed advertisements on radio and on social media in order to get people to use the stamps.