For the UK the change involves the addition of 978 in front of the old ISBN and the re-calculation of the final digit of the number. This final digit is actually a check digit which is calculated from all the previous numbers and is used to ensure that the ISBN is valid. So in most cases the new check digit will be different from the last digit in the old ISBN.
It will probably take some time before everyone is only using the new system so if you want to order a book through a library or bookshop it will probably be wise to make a note of both numbers.
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