The Key of Changes shows the 8-digit AHECC codes that have been replaced or added to the classification. It is a historical concordance between old and new codes. The attached Acrobat file contains a listing of all changes to AHECC codes that have occurred since the original version of the classification (produced on 1 January 1988) including changes introduced up till 1 January 2006.
The list has four columns:
| Previous code | The code that has closed |
| Replacement code | The new code(s) that replaced a previous (or closed) code. |
| Date replaced | The date a previous (or closed) code ceased to operate, and was replaced by its new code(s). |
| Expiry date | The date a replacement code operated until, before it was replaced by a new code. |
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