The primary key field should be a text field regardless, with an auto-enter option set to Get ( UUID ). If you turn the auto-enter option off to perform your import, then the existing key values will be imported and other auto-enter options will still be performed on import, such as creation timestamps and the like. After the import all you need to do to go back to normal is to turn back on the auto-enter option on your primary key field. It sounds to me as if you might have changed the field to a calculation field instead.
Yes, that's what I was doing.
Thank-you for the fix.