I think maybe you need to read the link stuff again for a fuller understanding.
Changing mailboxes into CRs is slightly problematic as they have a slightly different set of fields - for instance, the 'e-mail address' (singular) is vital for a CR, whereas for a mailbox it's blank, but then you need to consider the 'e-mail addresses' (plural) field.
Explorting the current CRs is a good way of getting a default set of headers (although by no means necessarily all the fields they have set), but you don't need to include the existing CRs in the file when you reimport (unless you're changing any of their fields). It is possible to simply import an exported file, but I would urge you to read up on and use the Mode field (has to be the second column, after Obj-Class), it can save you some difficulties.