I have added this to the general summary post on the Swap-Out system, it is worth repeating here to bring to your attention two important aspects.
There have been suggestions that Post Offices will not accept letters and packets with non-barcoded stamps after 31 January 2023. I asked about this in my Crown PO today and was told that this was not true: they have been specifically told that they should continue to accept them as normal.
On the other hand the Royal Mail Special Handstamp Centres will not accept items for postmarking with a date after 31 January which have non-barcoded definitives on.
UPDATE 31 January. As the official validity date of non-barcode Machin and Country Definitives is now the 31st July 2023, we can confirm that these stamps will continue to be accepted by our Special Handstamp Centres until that date.
Further thoughts, on the effect of changes in postage rates on the Swap-Out scheme have been posted on the summary thread.