I had this happen with two houses that I lived in. The first was an existing house, built maybe 5 years before I bought it. The postcode digit in the second part changed some time during the 1990s from 3 to 0. The Royal Mail were no help at all: their attitude was "your postcode has changed - if any letters are addressed to the old postcode they will not be delivered". I imagine many people's postcodes were affected in the same way - a change of digit but the letters remained the same: xx12 3yy went to xx12 0yy.
Then the same thing happened in my second house - one of a brand new development. All the houses had the same code and the digit changed from 5 to 4. I'm not sure whether the builder had been given the wrong information or whether the Royal Mail had allocated a code in the wrong area.
In both cases, the old postcode was never reused so it would have been easy enough for the Royal Mail to have set up an automatic redirection to give people chance to wrote to all the many companies to get the postcode changed on their database.