Because when you change the address, you are changing the address pointed to by the code in the older PO's. Only the code is stored. The address on the form is the address retrieved from the file as it currently exists, even for an old PO.
Just make up a new code for each address. There is no practical limit on the number of codes.
We are currently re-writing Purchase Orders for Adagio, but this is not one of the changes that will be in first version. we want both the old and new versions to run beside each other without converting the data, so that people can e confident that the new version is working well without disrupting their business.
Once we're confident that we're there, then we'll be happy to introduce other changes that would be useful to people.
This would make a good "Adagio Wish"!
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Andrew Bates